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How to Solve Linear Differential Equation

A linear equation or polynomial, with one or more terms, consisting of the derivatives of the dependent variable with respect to one or more independent variables is known as a linear differential equation.

A general first-order differential equation is given by the expression:

dy/dx + Py = Q where y is a function and dy/dx is a derivative.

The solution of the linear differential equation produces the value of variable y.

Linear Differential Equations Definition

A linear differential equation is defined by the linear polynomial equation, which consists of derivatives of several variables. It is also stated as Linear Partial Differential Equation when the function is dependent on variables and derivatives are partial.

A differential equation having the above form is known as the first-order linear differential equation  where P and Q are either constants or functions of the independent variable (in this case x) only.

Also, the differential equation of the form,  dy/dx + Py = Q , is a   first-order linear differential equation where P and Q are either constants or functions of y (independent variable) only.

To find linear differential equations solution, we have to derive the general form or representation of the solution.

Linear Differential Equation Solution

Non-Linear Differential Equation

When an equation is not linear in unknown function and its derivatives, then it is said to be a nonlinear differential equation. It gives diverse solutions which can be seen for chaos. 

Solving Linear Differential Equations

For finding the solution of such linear differential equations, we determine a function of the independent variable let us say M(x), which is known as the Integrating factor (I.F).

Multiplying both sides of equation (1) with the integrating factor M(x) we get;

M(x)dy/dx + M(x)Py = QM(x)  …..(2)

Now we chose M(x) in such a way that the L.H.S of equation (2) becomes the derivative of y.M(x)

i.e.    d(yM(x))/dx = (M(x))dy/dx + y (d(M(x)))dx … (Using d(uv)/dx   = v(du/dx)   + u(dv/dx)

⇒ M(x) /(dy/dx) + M(x)Py = M (x) dy/dx + y d(M(x))/dx

⇒ M(x)Py = y dM(x)/dx

⇒ 1/M'(x) = P.dx 

Integrating both sides with respect to x, we get;

\(\begin{array}{l} log M (x) = \int Pdx (As \int \frac {f'(x)}{f(x)} ) = log f(x) \end{array} \)

\(\begin{array}{l} M(x) = e^{\int Pdx}= I.F\end{array} \)

Now, using this value of the integrating factor, we can find out the solution of our first order linear differential equation.

Multiplying both the sides of equation (1) by the I.F. we get

\(\begin{array}{l} e^{\int Pdx}\frac{dy}{dx} + yPe^{\int Pdx} = Qe^{\int Pdx} \end{array} \)

This could be easily rewritten as:

\(\begin{array}{l} \frac {d(y.e^{\int Pdx})}{dx} = Qe^{\int Pdx} (Using \frac{d(uv)}{dx} = v \frac{du}{dx} + u\frac{dv}{dx} ) \end{array} \)

Now integrating both the sides with respect to x, we get:

\(\begin{array}{l} \int d(y.e^{\int Pdx }) = \int Qe^{\int Pdx}dx + c \end{array} \)

\(\begin{array}{l} y = \frac {1}{e^{\int Pdx}} (\int Qe^{\int Pdx}dx + c )\end{array} \)

where C is some arbitrary constant.

How to Solve First Order Linear Differential Equation

Learn to solve the first-order differential equation with the help of steps given below.

          where P and Q are constants or functions of the independent variable x only.

\(\begin{array}{l} e^{\int Pdx} = I.F\end{array} \)

\(\begin{array}{l} e^{\int Pdx} \frac{dy}{dx} + yPe^{\int Pdx} = Qe^{\int Pdx} \end{array} \)

i.e. L.H.S = d(y × I.F)/dx

d(y × I.F)dx = Q × I.F 

where C is some arbitrary constant

Similarly, we can also solve the other form of linear first-order differential equation dx/dy +Px = Q using the same steps. In this form P and Q are the functions of y. The integrating factor (I.F) comes out to be  and using this we find out the solution which will be

Now, to get a better insight into the linear differential equation, let us try solving some questions. where C is some arbitrary constant.

Solved Examples

Example 1: Solve the  LDE =   dy/dx = [1/(1+x 3 )] – [3x 2 /(1 + x 2 )]y

The above mentioned equation can be rewritten as   dy/dx + [3x 2 /(1 + x 2 )] y = 1/(1+x 3 )

Comparing it with  dy/dx + Py = O , we get

P = 3x 2 /1+x 3

Q= 1/1 + x 3

Let’s figure out the integrating factor(I.F.) which is,

\(\begin{array}{l} e^{\int Pdx} \end{array} \)

\(\begin{array}{l}I.F  = e^{\int \frac {3x^2}{1 + x^3}} dx = e^{ln (1 + x^3)} \end{array} \)

⇒I.F. = 1 + x 3

Now, we can also rewrite the L.H.S as:

d(y × I.F)/dx, 

⇒ d(y × (1 + x 3 )) dx = [1/(1 +x 3 )] × (1 + x 3 )

Integrating both the sides w. r. t. x, we get,

⇒ y ×  ( 1 + x 3 ) =  x

⇒ y = x/(1 + x 3 )

⇒ y = [ x/(1 + x 3  ) +  C

Solve the following differential equation:  

dy/dx + (sec x)y = 7 

Comparing the given equation with dy/dx + Py = Q 

We see,  P = sec  x, Q = 7

Now lets find out the integrating factor using the formula

\(\begin{array}{l} e^{\int Pdx}= I.F \end{array} \)

\(\begin{array}{l} e^{\int secdx}= I.F. \end{array} \)

\(\begin{array}{l} I.F. = e^{ln |sec x + tan x |} = sec x + tan x  \end{array} \)

Now we can also rewrite the L.H.S as

d(y × I.F)/dx} ,

i.e . d(y × (sec x + tan x ))

⇒ d(y × (sec x + tan x ))/dx = 7(sec x + tan x) 

\(\begin{array}{l}  \int d ( y × (sec x + tan x ))  = \int 7(sec x + tan x) dx \end{array} \)

\(\begin{array}{l} \Rightarrow y × (sec x + tan x) = 7 (ln|sec x + tan x| + log |sec x| ) \end{array} \)

\(\begin{array}{l} y =\frac {7(ln|sec x + tan x| + log|sec x| }{(sec x + tan x)} + c \end{array} \)

A curve is passing through the origin and the slope of the tangent at a point R(x,y) where -1<x<1 is given as ( x 4 + 2xy + 1)/(1 – x 2 ) . What will be the equation of the curve?

We know that the slope of the tangent at (x,y) is,

tanƟ= dy/dx = (x 4 + 2xy + 1)/1 – x 2

Reframing the equation in the form dy/dx  + Py = Q  , we get

dy/dx = 2xy/(1 – x 2 ) + (x 4 + 1)/(1 – x 2 )

⇒ dy/dx – 2xy/(1 – x 2 ) = (x 4 + 1)/(1 – x 2 )

Comparing we get P = -2x/(1 – x 2 )

Q = ( x 4 + 1)/(1 – x 2 )

Now, let’s find out the integrating factor using the formula.

\(\begin{array}{l} e^{\int \frac{-2x}{1-x^2}}dx = e^{ln (1 – x^2)} = 1 – x^2 =I.F \end{array} \)

\(\begin{array}{l} \frac {d(y × I.F)}{dx}, \end{array} \)

\(\begin{array}{l} i.e.,\frac{d(y × (1 – x^2))}{dx} = \frac{x^4 + 1}{1 – x^2} × 1 – x^2 \end{array} \)

Integrating both sides w. r. t. x, we get,

\(\begin{array}{l} \int d(y × (1 – x^2)) = \int \frac{x^4 + 1}{1 – x^2} × (1 – x^2 )dx \end{array} \)

\(\begin{array}{l} \Rightarrow y × (1 – x^2) = \int x^4 + 1 dx …(1) \end{array} \)

 x (1 – x 2 ) = x 5 /5 + x + C

⇒ y =  x 5 /5 + x/(1 – x 2 ) + C 

It is the required equation of the curve. Also as the curve passes through origin; substitute the values as x = 0, y = 0 in the above equation. Thus, C = 0.

Frequently Asked Questions – FAQs

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Section 2.1 : Linear Differential Equations

The first special case of first order differential equations that we will look at is the linear first order differential equation. In this case, unlike most of the first order cases that we will look at, we can actually derive a formula for the general solution. The general solution is derived below. However, we would suggest that you do not memorize the formula itself. Instead of memorizing the formula you should memorize and understand the process that I'm going to use to derive the formula. Most problems are actually easier to work by using the process instead of using the formula.

So, let's see how to solve a linear first order differential equation. Remember as we go through this process that the goal is to arrive at a solution that is in the form \(y = y\left( t \right)\). It's sometimes easy to lose sight of the goal as we go through this process for the first time.

In order to solve a linear first order differential equation we MUST start with the differential equation in the form shown below. If the differential equation is not in this form then the process we’re going to use will not work.

Where both \(p(t)\) and \(g(t)\) are continuous functions. Recall that a quick and dirty definition of a continuous function is that a function will be continuous provided you can draw the graph from left to right without ever picking up your pencil/pen. In other words, a function is continuous if there are no holes or breaks in it.

Now, we are going to assume that there is some magical function somewhere out there in the world, \(\mu \left( t \right)\), called an integrating factor . Do not, at this point, worry about what this function is or where it came from. We will figure out what \(\mu \left( t \right)\) is once we have the formula for the general solution in hand.

So, now that we have assumed the existence of \(\mu \left( t \right)\) multiply everything in \(\eqref{eq:eq1}\) by \(\mu \left( t \right)\). This will give.

Now, this is where the magic of \(\mu \left( t \right)\) comes into play. We are going to assume that whatever \(\mu \left( t \right)\) is, it will satisfy the following.

Again do not worry about how we can find a \(\mu \left( t \right)\) that will satisfy \(\eqref{eq:eq3}\). As we will see, provided \(p(t)\) is continuous we can find it. So substituting \(\eqref{eq:eq3}\) we now arrive at.

At this point we need to recognize that the left side of \(\eqref{eq:eq4}\) is nothing more than the following product rule.

So we can replace the left side of \(\eqref{eq:eq4}\) with this product rule. Upon doing this \(\eqref{eq:eq4}\) becomes

Now, recall that we are after \(y(t)\). We can now do something about that. All we need to do is integrate both sides then use a little algebra and we'll have the solution. So, integrate both sides of \(\eqref{eq:eq5}\) to get.

Note the constant of integration, \(c\), from the left side integration is included here. It is vitally important that this be included. If it is left out you will get the wrong answer every time.

The final step is then some algebra to solve for the solution, \(y(t)\).

Now, from a notational standpoint we know that the constant of integration, \(c\), is an unknown constant and so to make our life easier we will absorb the minus sign in front of it into the constant and use a plus instead. This will NOT affect the final answer for the solution. So with this change we have.

Again, changing the sign on the constant will not affect our answer. If you choose to keep the minus sign you will get the same value of \(c\) as we do except it will have the opposite sign. Upon plugging in \(c\) we will get exactly the same answer.

There is a lot of playing fast and loose with constants of integration in this section, so you will need to get used to it. When we do this we will always to try to make it very clear what is going on and try to justify why we did what we did.

So, now that we’ve got a general solution to \(\eqref{eq:eq1}\) we need to go back and determine just what this magical function \(\mu \left( t \right)\) is. This is actually an easier process than you might think. We’ll start with \(\eqref{eq:eq3}\).

Divide both sides by \(\mu \left( t \right)\),

Now, hopefully you will recognize the left side of this from your Calculus I class as nothing more than the following derivative.

As with the process above all we need to do is integrate both sides to get.

You will notice that the constant of integration from the left side, \(k\), had been moved to the right side and had the minus sign absorbed into it again as we did earlier. Also note that we’re using \(k\) here because we’ve already used \(c\) and in a little bit we’ll have both of them in the same equation. So, to avoid confusion we used different letters to represent the fact that they will, in all probability, have different values.

Exponentiate both sides to get \(\mu \left( t \right)\) out of the natural logarithm.

Now, it’s time to play fast and loose with constants again. It is inconvenient to have the \(k\) in the exponent so we’re going to get it out of the exponent in the following way.

Now, let’s make use of the fact that \(k\) is an unknown constant. If \(k\) is an unknown constant then so is \({{\bf{e}}^k}\) so we might as well just rename it \(k\) and make our life easier. This will give us the following.

So, we now have a formula for the general solution, \(\eqref{eq:eq7}\), and a formula for the integrating factor, \(\eqref{eq:eq8}\). We do have a problem however. We’ve got two unknown constants and the more unknown constants we have the more trouble we’ll have later on. Therefore, it would be nice if we could find a way to eliminate one of them (we’ll not be able to eliminate both….).

This is actually quite easy to do. First, substitute \(\eqref{eq:eq8}\) into \(\eqref{eq:eq7}\) and rearrange the constants.

So, \(\eqref{eq:eq7}\) can be written in such a way that the only place the two unknown constants show up is a ratio of the two. Then since both \(c\) and \(k\) are unknown constants so is the ratio of the two constants. Therefore we’ll just call the ratio \(c\) and then drop \(k\) out of \(\eqref{eq:eq8}\) since it will just get absorbed into \(c\) eventually.

The solution to a linear first order differential equation is then

Now, the reality is that \(\eqref{eq:eq9}\) is not as useful as it may seem. It is often easier to just run through the process that got us to \(\eqref{eq:eq9}\) rather than using the formula. We will not use this formula in any of our examples. We will need to use \(\eqref{eq:eq10}\) regularly, as that formula is easier to use than the process to derive it.

Solution Process

The solution process for a first order linear differential equation is as follows.

Let’s work a couple of examples. Let’s start by solving the differential equation that we derived back in the Direction Field section.

First, we need to get the differential equation in the correct form.

From this we can see that \(p(t)=0.196\) and so \(\mu \left( t \right)\) is then.

Note that officially there should be a constant of integration in the exponent from the integration. However, we can drop that for exactly the same reason that we dropped the \(k\) from \(\eqref{eq:eq8}\).

Now multiply all the terms in the differential equation by the integrating factor and do some simplification.

Integrate both sides and don't forget the constants of integration that will arise from both integrals.

Okay. It’s time to play with constants again. We can subtract \(k\) from both sides to get.

Both \(c\) and \(k\) are unknown constants and so the difference is also an unknown constant. We will therefore write the difference as \(c\). So, we now have

From this point on we will only put one constant of integration down when we integrate both sides knowing that if we had written down one for each integral, as we should, the two would just end up getting absorbed into each other.

The final step in the solution process is then to divide both sides by \({{\bf{e}}^{0.196t}}\) or to multiply both sides by \({{\bf{e}}^{ - 0.196t}}\). Either will work, but we usually prefer the multiplication route. Doing this gives the general solution to the differential equation.

From the solution to this example we can now see why the constant of integration is so important in this process. Without it, in this case, we would get a single, constant solution, \(v(t)=50\). With the constant of integration we get infinitely many solutions, one for each value of \(c\).

Back in the direction field section where we first derived the differential equation used in the last example we used the direction field to help us sketch some solutions. Let's see if we got them correct. To sketch some solutions all we need to do is to pick different values of \(c\) to get a solution. Several of these are shown in the graph below.

A graph with domain $0 \le t \le 10$ and range $0 \le v(t) \le 90$.  There are a series of solutions graphed corresponding to values of c.  The graph for c=0 is a horizontal line at v=50.  The graphs for c=-20 and c=-40 are below the graph of c=0 and increase up towards the c=0 graph.  They shallow out as the near v=50 and don’t cross the horizontal line.  The graph of c=-40 is below the graph of c=-20. The graphs for c=20 and c=40 are above the graph of c=0 and decrease down towards the c=0 graph.  They shallow out as the near v=50 and don’t cross the horizontal line.  The graph of c=40 is above the graph of c=20.

So, it looks like we did pretty good sketching the graphs back in the direction field section.

Now, recall from the Definitions section that the Initial Condition(s) will allow us to zero in on a particular solution. Solutions to first order differential equations (not just linear as we will see) will have a single unknown constant in them and so we will need exactly one initial condition to find the value of that constant and hence find the solution that we were after. The initial condition for first order differential equations will be of the form

Recall as well that a differential equation along with a sufficient number of initial conditions is called an Initial Value Problem (IVP).

To find the solution to an IVP we must first find the general solution to the differential equation and then use the initial condition to identify the exact solution that we are after. So, since this is the same differential equation as we looked at in Example 1 , we already have its general solution.

Now, to find the solution we are after we need to identify the value of \(c\) that will give us the solution we are after. To do this we simply plug in the initial condition which will give us an equation we can solve for \(c\). So, let's do this

So, the actual solution to the IVP is.

A graph of this solution can be seen in the figure above.

Let’s do a couple of examples that are a little more involved.

Rewrite the differential equation to get the coefficient of the derivative to be one.

Now find the integrating factor.

Can you do the integral? If not rewrite tangent back into sines and cosines and then use a simple substitution. Note that we could drop the absolute value bars on the secant because of the limits on \(x\). In fact, this is the reason for the limits on \(x\). Note as well that there are two forms of the answer to this integral. They are equivalent as shown below. Which you use is really a matter of preference.

Also note that we made use of the following fact.

This is an important fact that you should always remember for these problems. We will want to simplify the integrating factor as much as possible in all cases and this fact will help with that simplification.

Now back to the example. Multiply the integrating factor through the differential equation and verify the left side is a product rule. Note as well that we multiply the integrating factor through the rewritten differential equation and NOT the original differential equation. Make sure that you do this. If you multiply the integrating factor through the original differential equation you will get the wrong solution!

Integrate both sides.

Note the use of the trig formula \(\sin \left( {2\theta } \right) = 2\sin \theta \cos \theta \) that made the integral easier. Next, solve for the solution.

Finally, apply the initial condition to find the value of \(c\).

The solution is then.

Below is a plot of the solution.

A graph with domain $0 \le x \le 20$ and range $-6 \le y \le 6$.  The graph is essentially the graph of a cosine function with an amplitude of 6 and a wavelength of approximately 6.

First, divide through by the t to get the differential equation into the correct form.

Now let’s get the integrating factor, \(\mu \left( t \right)\).

Now, we need to simplify \(\mu \left( t \right)\). However, we can’t use \(\eqref{eq:eq11}\) yet as that requires a coefficient of one in front of the logarithm. So, recall that

and rewrite the integrating factor in a form that will allow us to simplify it.

We were able to drop the absolute value bars here because we were squaring the \(t\), but often they can’t be dropped so be careful with them and don’t drop them unless you know that you can. Often the absolute value bars must remain.

Now, multiply the rewritten differential equation (remember we can’t use the original differential equation here…) by the integrating factor.

Integrate both sides and solve for the solution.

Finally, apply the initial condition to get the value of \(c\).

The solution is then,

Here is a plot of the solution.

A graph with domain $0 \le x \le 20$ and range $0 \le y \le 25$.  The starts at approximately (0,25) and decreases sharply until reaching approximately (0.1,1) and then starts to increase for the rest of the graph.  The rate of increase is shallow at first then steepens out until it seems to be at approximately a 45 degree angle with the x-axis at the right end of the graph.

First, divide through by \(t\) to get the differential equation in the correct form.

Now that we have done this we can find the integrating factor, \(\mu \left( t \right)\).

Do not forget that the “-” is part of \(p(t)\). Forgetting this minus sign can take a problem that is very easy to do and turn it into a very difficult, if not impossible problem so be careful!

Now, we just need to simplify this as we did in the previous example.

Again, we can drop the absolute value bars since we are squaring the term.

Now multiply the differential equation by the integrating factor (again, make sure it’s the rewritten one and not the original differential equation).

Apply the initial condition to find the value of \(c\).

The solution is then

A graph with domain $0 \le x \le 7$ and range $0 \le y \le 5000$.  The starts at approximately (1,0) has a very shallow increasing slope.  Between $3<x<6$ the graph steepens somewhat and starting at x=6 it sharply increases.

Let’s work one final example that looks more at interpreting a solution rather than finding a solution.

First, divide through by a 2 to get the differential equation in the correct form.

Now find \(\mu \left( t \right)\).

Multiply \(\mu \left( t \right)\)through the differential equation and rewrite the left side as a product rule.

Integrate both sides (the right side requires integration by parts – you can do that right?) and solve for the solution.

Apply the initial condition to find the value of \(c\) and note that it will contain \(y_{0}\) as we don’t have a value for that.

So, the solution is

Now that we have the solution, let’s look at the long term behavior ( i.e. \(t \to \infty \)) of the solution. The first two terms of the solution will remain finite for all values of \(t\). It is the last term that will determine the behavior of the solution. The exponential will always go to infinity as \(t \to \infty \), however depending on the sign of the coefficient \(c\) (yes we’ve already found it, but for ease of this discussion we’ll continue to call it \(c\)). The following table gives the long term behavior of the solution for all values of \(c\).

This behavior can also be seen in the following graph of several of the solutions.

A graph with domain $0 \le t \le 8$ and range $-8 \le y \le 8$.  There are a series of graph representing different values of c on the graph.  The c=0 graph is essentially an oscillation along the t-axis.  Graphs corresponding to c>0 (there are 5 of them with no actual c values indicated) all start with y>0 and increase and have an oscillation to them as the increase.  The farther up the y axis they start the faster they increase.  Graphs corresponding to c<0 (there are 5 of them with no actual c values indicated) all start with y<0 and decrease and have an oscillation to them as the decrease.  The farther down the y axis they start the faster they decrease.

Now, because we know how \(c\) relates to \(y_{0}\) we can relate the behavior of the solution to \(y_{0}\). The following table give the behavior of the solution in terms of \(y_{0}\) instead of \(c\).

Note that for \({y_0} = - \frac{{24}}{{37}}\) the solution will remain finite. That will not always happen.

Investigating the long term behavior of solutions is sometimes more important than the solution itself. Suppose that the solution above gave the temperature in a bar of metal. In this case we would want the solution(s) that remains finite in the long term. With this investigation we would now have the value of the initial condition that will give us that solution and more importantly values of the initial condition that we would need to avoid so that we didn’t melt the bar.

Linear Differential Equation

Linear differential equation is an equation having a variable, a derivative of this variable, and a few other functions. The standard form of a linear differential equation is dy/dx + Py = Q, and it contains the variable y, and its derivatives. The P and Q in this differential equation are either numeric constants or functions of x.

The linear differential equation in an important form of a differential equation and can be solved using a formula. Let us learn the formula and derivation, to find the general solution of a linear differential equation.

What Is a Linear Differential Equation?

The linear differential equation is of the form dy/dx + Py = Q, where P and Q are numeric constants or functions in x. It consists of a y and a derivative of y. The differential is a first-order differentiation and is called the first-order linear differential equation.

This linear differential equation is in y. Similarly, we can write the linear differential equation in x also. The linear differential equation in x is dx/dy + \(P_1\)x = \(Q_1\).

Linear Differential Equations in x and y

Some of the examples of linear differential equation in y are dy/dx + y = Cosx, dy/dx + (-2y)/x = x 2 .e -x . And the examples of linear differential equation in x are dx/dy + x = Siny, dx/dy + x/y = ey. dx/dy + x/(ylogy) = 1/y.

Derivation for Solution of Linear Differential Equation

The derivation for the general solution for the linear differential equation can be understood through the below sequence of steps. The first-order differential equation is of the form.

dy/dx +Px = Q

Here we multiply both sides of the equation by a function of x, say g(x) . Further, this function is chosen such that the right hand side of the equation is derivative of y.g(x). d/dx(y.g(x)) = y.g(x).

g(x).dy/dx + P.g(x).y = Q.g(x)

Choose g(x) in such a way such that the RHS becomes the derivative of y.g(x).

g(x).dy/dx + P.g(x)y = d/dx(y.g(x)]

The right hand side of the above expression is derived using the derivative formula for the product of functions.

g(x).dy/dx + P.g(x).y = g(x).dy/dx + y.g'(x)

P.g(x) = g'(x)

P = g'(x)/g(x)

Integrating both sides with respect to x, we get

\(\int P.dx= \int \frac{g'(x)}{g(x)}.dx\)

\(\int P.dx= log(g(x))\)

\(g(x)= e^{\int P.dx}\)

This function \(g(x)= e^{\int P.dx}\) is called the Integrating Factor (I.F) of the given linear differential equation. Substituting the value of g(x) in equation linear differential equation, the following expression is obtained.

\(e^{\int P.dx}.\dfrac{dy}{dx} + Pe^{\int P.dx}y = Q.e^{\int P.dx}\)

\(\dfrac{d}{dx}(y.e^{\int P.dx} )= Qe^{\int P.dx}\)

Integrating both sides, with respect to x the following expression is obtained..

\(y.e^{\int P.dx} =\int (Q.e^{\int P.dx}.dx)\)

\(y=e^{-\int P.dx} .\int (Q.e^{\int P.dx}.dx) + C\)

The above expression is the general solution of the linear differential eqution.

Formula for General Solution of Linear Differential Equation

The following are the two important formulas to find the general solution of the linear differential equations.

Steps to Solve Linear Differential Equation

The following three simple steps are helpful to write the general solutions of a linear differential equation.

The usage of the above steps can be more clearly understood through the below-solved examples of the linear differential equation.

Related Topics

Examples on Linear Differential Equation

Example 1: Find the general solution of the differential equation xdy -(y + 2x 2 ).dx = 0

Solution: The give differential equation is xdy - (y + 2x 2 ).dx = 0. This can be simplified to represent the following linear differential equation.

dy/dx - y/x = 2x

Comparing this with the differential equation dy/dx + Py = Q we have the values of P = -1/x and the value of Q = 2x. Hence we have the integration factor as IF = \(e^{\int -\dfrac{1}{x}.dx}\) = \(e^{-\log x}\) = \(\frac{1}{x}\).

Further, the solution of the differential equation is as follows.

y\(\frac{1}{x}\) = \(\int 2x.\frac{1}{y} .dx + c\) \(\frac{y}{x}\) = \(\int 2.dx + c\) \(\frac{y}{x}\) = 2x + c y = 2x 2 + xc Answer: Thus the general solution of the given linear differential equation is y = 2x 2 + xc

Example 2: Find the derivative of dy/dx + Secx.y = Tanx

The given differential equation is dy/dx + Secx.y = Tanx. Comparing this with the linear differential equation dy/dx + Px = Q, we have P = Secx, and Q = Tanx.

Further the integrating factor is I.F = \(e^{\int Secx.dx}

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example problem in linear differential equation

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Practice Questions on Linear Differential Equation

Faqs on linear differential equation.

The linear differential equation is an equation having a variable, a derivative of this variable, and a few other functions. The standard form of a linear differential equation is dy/dx + Py = Q, and it contains the variable y, and its derivatives. The P and Q in this differential equation are either numeric constants or functions of x.

How Do You Know If a Differential Equation Is a Linear Differential Equation?

A differential equation is said to be a linear differential equation if it has a variable and its first derivative. The linear differential equation in y is of the form dy/dx + Py = Q, Here we have the variable y, the first derivative of the variable y, and we have P, Q which are functions in x. From the name of linear, these differential equations have only the first degree derivatives.

How Do You Solve a Linear Differential Equation?

The solution of a linear differential equation is through three simple steps. First simplify and write the given differential equation in the form dy/dx + Py = Q. For this find the Integrating Factor (IF) = \(e^{\int P.dx}\). Finally the solution of the linear differential equation is \(y(I.F) = \int(Q × I.F).dx + C\)

What Is the Standard Form of Linear Differential Equation in x?

The standard form of the linear differential equation in x is dx/dy + Px = Q, This is a differential equation having a variable x, the first derivative of x, and P, Q represent the functions in y. The linear differential equation in x has first-order derivative of x.

What Is the Formula For the General Solution of Linear Differential Equation

The formula for general solution of the differential equation dy/x +Py = Q is \(y.(I.F)=\int (Q.(I.F).dx)+ C\). Here we have Integrating Factor (I.F) = \(e^{\int P.dx}\). Also the formula for the general solution of the differential equation dx/y +Px = Q is \(x.(I.F)=\int (Q.(I.F).dy)+ C\). Here we have Integrating Factor (I.F) = \(e^{\int P.dy}\).

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Linear vs nonlinear differential equation

How to distinguish linear differential equations from nonlinear ones? I know, that e.g.: $$ y''-2y = \ln(x) $$ is linear, but $$ 3+ yy'= x - y $$ is nonlinear. Why?

Ayman Hourieh's user avatar

5 Answers 5

Linear differential equations are those which can be reduced to the form $Ly = f$, where $L$ is some linear operator.

Your first case is indeed linear, since it can be written as:

$$\left(\frac{d^2}{dx^2} - 2\right)y = \ln(x)$$

While the second one is not. To see this first we regroup all $y$ to one side:

$$y(y'+1) = x - 3$$

then we simply notice that the operator $y\mapsto g(y) = y(y'+1)$ is not linear (for example we can take two functions $y_1$ and $y_2$ and notice that $g(y_1+y_2)\neq g(y_1) + g(y_2)$).

J. M. ain't a mathematician's user avatar

If the equation would have had $\ln (y)$ on the right, that also would have made it non-linear, since natural logs are non-linear functions. Remember that this has its roots in linear algebra: $y=mx+b$. You can analyse functions term-by-term to determine if they are linear, if that helps. The first time a term is non-linear, then the entire equation is non-linear.

Remember that the $x$s can pretty much do or appear however they want, since they're independent. Which means if you can't tell just by glancing, try to group all your $y$ terms to one side and then analyse them. Makes it much easier.

See, I was also overthinking this, but realised you have to go back to those definitions we're given.

Two criteria for linearity:

The dependent variable y and its derivatives are of first degree; the power of each y is 1. $\frac{dy}{dx}$; yes. $(\frac{dy}{dx})^4$, no.

Each coefficient depends only on the independent variable $x$.

$yy'$ makes it nonlinear as has been said, because that coefficient on $y'$ is not in $x$. Had that coefficient been a constant, you would have been correct to call it linear, since constants can be functions of $x$. Like, $f(3)=x$. Its graph is a line, i.e. linear function.

Always go back to the definitions. :-)

OpusCroakus's user avatar

One could define a linear differential equation as one in which linear combinations of its solutions are also solutions.

Geremia's user avatar

Linear Differential equations are those in which the dependent variable and its derivatives appear only in first degree and not multiplied together

John Leo's user avatar

Because highest order derivative is multiplied with dependent variable $y$. Like $y y'$.

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Linear differential equation problems and answers

21-22 Solve the differential equation and use a calcula tor to graph several members of the family of solutions. How does the solution curve change as C

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Linear Differential Equation

5.3 first order linear differential equations.

A linear differential equation is defined by the linear polynomial equation, which consists of derivatives of several variables. It is also stated as Linear

Linear Differential Equation (Solution & Solved Examples)

We now introduce the first one of two methods discussed in these notes to solve a first order non-homogeneous linear differential equation.

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Linear Differential Equation

A differential equation (an equation that connects one or more functions and their derivatives) interpreted through a linear expression that has 1 or more terms being algebraic in the function which is unknown and its derivatives is a linear differential equation in mathematics. It is of the numerical form 

Aforesaid equations are ordinary differential equations. If the function that is unknown is dependent on various variables & the derivatives present in the equation are derivatives that are partial in nature, then a linear differential equation is a linear partial differential equation. A linear differential equation or system of equations that are linear in a way that the related homogeneous equations consist of coefficients that are constant can be figured out by the method of quadrature, which implies the solutions can be represented in form of integrals. This holds good for the linear equation with order 1, with coefficients that are non-constant. A linear combination consisting of rudimentary differential operators, with coefficients as differentiable functions. The linear operator in the univariate scenario is given by 

Linear Differential Equation Formula

The standard form of a linear differential equation is (dy / dx) + Py = Q. Here P and Q are constants in x. It possesses the term y and its derivative. It is of first-order and hence termed first-order linear differential equation. The differential is in terms of y, similarly, it can be written in terms of x also. The linear differential equation in terms of x can be expressed as (dx/dy)+P_{1}x=Q_{1}

Linear Differential Equation Properties

The linear differential equations have the following properties.

a] The y function and its respective derivatives come in the equation till the first degree only.

b] The products of y and/or any of its respective derivatives are not present.

c] No functions that are transcendental. 

Linear Differential Equation Examples

Example 1: Solve the equation: x \frac{d y}{d x}-2 y=x^{3} \cos 4 x .

Answer: 

Convert the given equation into the standard form (dy / dx) + Py = Q of the linear differential equation. 

Obtain the values of P and Q by comparing it to the standard form of linear differential equation.

P = (- 2 / x) and Q = x^{2} \cos 4 x

Compute the integrating factor by using the below formula.

IF = η(𝑥) = e^{\int P(x)dx}

The general solution is y =\frac{\int \eta(x) Q(x) d x-C}{\eta(x)} \\ y =\frac{\int x^{-2} x^{2} \cos 4 x d x-C}{x^{-2}} \\ y =x^{2}\left(\frac{\sin 4 x}{4}-C\right)

Example 2: Solve the equation: y’ (x) + y (x) / x = 3x.

Example 3: Solve the equation: y^{\prime}-y-x e^{x}=0

Example 4: Solve the differential equation xy'=y+2x^{3}

This problem can be solved by using the method of variation of a constant. First, find the general solution of the homogeneous equation:

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Differential equations

First order differential equations, second order linear equations, laplace transform.

with Answer, Solution, Formula - Homogeneous Differential Equations: Solved Example Problems | 12th Business Maths and Statistics : Chapter 4 : Differential Equations

Chapter: 12th business maths and statistics : chapter 4 : differential equations, homogeneous differential equations: solved example problems.

Example 4.15

Solve the differential equation  y 2   dx   +  (  xy   +   x 2  ) dy   =  0

example problem in linear differential equation

Example 4.17

Find the particular solution of the differential equation  x   2   dy   +   y   (   x   +   y )   dx   =  0 given that  x   =  1,  y   =  1

example problem in linear differential equation

Example 4.18

If the marginal cost of producing  x  shoes is given by (3 xy   +   y 2  )  dx   +  ( x   2   +   xy )  dy   =  0 and the total cost of producing a pair of shoes is given by ₹12. Then find the total cost function.

Given marginal cost function is (x 2  + xy) dy + (3xy + y 2 )dx=0

example problem in linear differential equation

Example 4.19

example problem in linear differential equation

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Initial Value Problem Differential Equations

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What do the phrases "once upon a time", "in the beginning", and "on a dark and stormy night" all have in common? They all indicate the start of a story! That is what the initial value for a differential equation tells you, where to start. But unlike the fact that two stories that start with "once upon a time" aren't the same, all solutions to initial value problems might not be so unique. So read through to figure out how to make sure you get a unique solution to your problem!

Definition of an Initial-Value Problem

Let's start by answering a common question. Is it supposed to be "initial-value" or "initial value"? Well, it depends on what country you are in and who wrote the textbook you are reading! To be consistent this article doesn't use the hyphen. Instead initial value problem is abbreviated IVP , which gets around the question of whether that hyphen should be there or not.

So what is an IVP?

An IVP is a differential equation together with a place for a solution to start. They are often written

\[ \begin{align} &y' = f(x,y) \\ &y(a) = b \end{align} \]

where \((a,b)\) is the point the solution \(y(x)\) must go through.

Remember that the first step to solving a differential equation is to try and find the general solution, which gives you a constant of integration. Then using the initial value, you can find a particular solution. For more information on these topics see General Solutions to Differential Equations and Particular Solutions to Differential Equations.

First Order Differential Equation Initial Value Problem

Let's start with the constant coefficient first order linear differential equation

\[ y'+Ay=B\]

where \(A\) and \(B\) are real numbers with \(A \neq 0\). Remember that the general solution to this linear differential equation is

\[y(x) =Ce^{-Ax}+\frac{B}{A}\]

where \(C\) is a constant.

If you add the initial value \(y(x_1) = y_1\) where \(x_1\) and \(y_1\) are real numbers, then you can plug those into the general solution to get

\[ y_1=Ce^{-Ax_1}+\frac{B}{A}, \]

\[ C = \frac{y_1 - \frac{B}{A}}{e^{-Ax_1}}. \]

That means the solution to the IVP

\[ \begin{align} y' &= f(x,y) \\ y(x_1) &= y_1 \end{align} \]

\[ \begin{align} y(x) &= \frac{y_1 - \frac{B}{A}}{e^{-Ax_1}} e^{-Ax}+\frac{B}{A} \\ &= \left(y_1 - \frac{B}{A} \right)e^{-A(x-x_1)} +\frac{B}{A} . \end{align}\]

That can be a bit of a pain to memorize, so rather than doing that it is best to remember the general solution and then solve given your initial values. Let's take a look at an example.

Solve the IVP

\[ \begin{align} &y' +3y = 7 \\ &y(0) = \frac{1}{3}. \end{align} \]

The first step is finding the general solution, which for this problem is

\[y(x) =Ce^{-3x}+\frac{7}{3}.\]

Remember that this is actually a family of functions, and you can graph them for various values of \(C\).

The IVP is asking you to pick out the function that goes through the point \(\left(0, \frac{1}{3}\right)\). Using the initial value to solve for \(C\), you get

\[\frac{1}{3} =Ce^{-3\cdot 0}+\frac{7}{3},\]

so \(C = -2\). That means the solution to the IVP is

\[y(x) = -2e^{-3x}+\frac{7}{3}.\]

It is one particular function out of the family of functions that has the property that it goes through the initial value, as you can see in the graph below.

Solutions to linear constant coefficient first order differential equation IVPs have some nice properties that all come directly from the fact that you can explicitly write down the solution as

\[ y(x) = \left(y_1 - \frac{B}{A} \right)e^{-A(x-x_1)} +\frac{B}{A}.\]

Those properties are,

Solutions to the IVP are unique . This means that for every initial value you get one, and only one, value of \(C\) that works.

If you have two different initial values, the solutions corresponding to those initial values cannot cross. If they could cross it would violate the uniqueness part.

Solutions exist on the whole real line.

Solutions all have the same long term behavior. In other words, for a solution \(y(x)\) of the IVP, \[ \lim\limits_{x \to \infty} y(x) =\begin{cases} \frac{B}{A} & \mbox{ if } A>0 \\ \infty & \mbox{ if } A<0\quad \mbox{provided that } y_1-\frac{B}{A}>0 \\ -\infty & \mbox{ if } A<0\quad \mbox{provided that } y_1-\frac{B}{A}<0\end{cases}. \]

Now let's move on to the first order linear differential equation

\[ y'+P(x)y=Q(x)\]

where \(P(x)\) and \(Q(x)\) are functions. To find the solution to this differential equation you use the integrating factor

\[ \alpha(x)=e^{\int P(x)\,\mathrm{d}x},\]

and the general solution is

\[ y(x) = \frac{1}{\alpha (x)} \left( \int \alpha(x) \, Q(x) \, \mathrm{d}x + C \right).\]

Finding the solution to the IVP

\[ \begin{align} y'+P(x)y&=Q(x) \\ y(a) &= b \end{align} \]

isn't going to be nearly as nice as in the constant coefficient case since you need to do the integration before you can figure out what \(C\) is. So let's look at an example.

\[ \begin{align} &y'+\frac{1}{x}y=\frac{1}{\sqrt{x}} \\ &y(1) = \frac{5}{3} .\end{align} \]

First let's find the general solution. The integrating factor is

\[ \begin{align} \alpha(x)&=e^{\int P(x)\,\mathrm{d}x} \\ &= \exp\left( \int \frac{1}{x} \,\mathrm{d}x \right) \\ &= |x|. \end{align} \]

Notice that the initial value has \(x=1\), so in other words you are interested in positive values for \(x\) and you can drop the absolute value in the integrating factor to write

\[ \alpha(x) = x.\]

Then the general solution is given by

\[ \begin{align} y(x) &= \frac{1}{\alpha (x)} \left( \int \alpha(x) \, Q(x) \, \mathrm{d}x + C \right) \\ &= \frac{1}{x} \left( \int x \frac{1}{\sqrt{x}} \, \mathrm{d}x + C \right) \\ &= \frac{1}{x} \left( \int \sqrt{x} \, \mathrm{d}x + C \right) \\ &= \frac{1}{x} \left( \frac{2}{3}x^{\frac{3}{2} }+ C \right) \\ &= \frac{2\sqrt{x}}{3} + \frac{C}{x}. \end{align}\]

Now you can plug in the initial value \( y(1) = \dfrac{5}{3} \) to get

\[ \frac{5}{3} = \frac{2\sqrt{1}}{3} + \frac{C}{1}, \]

\[C = \frac{5}{3} - \frac{2}{3} = 1.\]

With the assumption that \(x>0\), the solution to the IVP is

\[ y(x) = \frac{2\sqrt{x}}{3} + \frac{1}{x}.\]

Notice that there was only one solution, but it certainly doesn't exist on the whole real line! For examples of a first order linear IVP without a solution, or with infinitely many solutions, take a look at Particular Solutions to Differential Equations .

You might wonder when a first order linear IVP will have a unique solution that exists on the whole real line.

If \(a, b \in \mathbb{R}\), and \(P(x)\), \(Q(x)\) are both continuous functions on the whole real line then the solution to the initial value problem

\[\begin{align} &y' + P(x)y = Q(x) \\ &y(a) = b \end{align}\]

exists and is unique on the whole real line .

Initial value Problems and Separable Differential Equations

Remember that a differential equation is separable if you can write it in the form

\[N(y)y' = M(x)\]

where \(N(y)\) and \(M(x)\) are functions. For more information and examples of this kind of equation see Separable Equations .

To make this into an IVP, all you need to do is pick an initial value. So for real numbers \(a\) and \(b\), the IVP is

\[\begin{align} &N(y)y' = M(x) \\ &y(a) = b. \end{align}\]

With separable equations, you often need to be careful with the interval of existence for solutions. In cases like that, the initial value tells you which of the intervals to choose as the interval of existence.

Let's take a look at an example.

If possible, solve the IVP

\[ \begin{align} &y' = \frac{y^2}{x} \\ &y(0) = 4. \end{align} \]

First, you can rewrite the differential equation as

\[ \frac{1}{y^2} y' = \frac{1}{x}, \]

so it is a separable differential equation. Separating variables and integrating gives you

\[ \int \frac{1}{y^2} \, \mathrm{d}y = \int \frac{1}{x} \, \mathrm{d}x\]

\[ -\frac{1}{y} = \ln |x| + C.\]

Now let's try and use the initial conditions. If you do, you will be using \(x=0\), and you can't take the natural log of zero. So in fact this IVP has no solution.

One more example, to see the kinds of things that can happen.

Consider the IVP

\[ \begin{align} &y' = y^{\frac{1}{3}} \\ &y(0) = 0. \end{align} \]

First show that the constant solution \(y(x)=0\) satisfies this IVP. Then see if there are any other solutions.

Certainly if you take the derivative of a constant function you get zero, and \(0^\frac{1}{3} = 0\), so the constant function \(y(x) = 0\) satisfies the differential equation. It also satisfies the initial condition, so it does satisfy the IVP.

What about other solutions? This is a separable equation, so separating and integrating gives you

\[ \int \frac{1}{y^\frac{1}{3} } \, \mathrm{d}y = \int 1\, \mathrm{d}x\]

\[ \frac{3}{2}y^\frac{2}{3} = x+ C.\]

Using the initial value \(y(0) = 0\) to find \(C\), you get

\[ \frac{3}{2}0^\frac{2}{3} = 0+ C,\]

so \(C=0\). That means there is a second solution to this IVP, namely the implicit solution

\[ \frac{3}{2}y^\frac{2}{3} = x.\]

You can get the explicit solution by solving for \(y\). If you do, you get

\[ y^\frac{2}{3} = \frac{2}{3}x,\]

\[ y(x) =\left( \frac{2}{3}x \right)^{\frac{3}{2}}.\]

Notice that the maximal interval of existence is \( [0,\infty) \) since you can't take the square root of a negative number. If you graph the two functions, you can see that both the constant function and the function \(y(x)\) that you solved for both satisfy the equation and the initial value.

More examples are always good!

Examples of Differential Equation Initial Value Problems

Let's look more examples involving IVPs.

Find the solution to the IVP

\[ \begin{align} &2xy'+4y=3 \\ &y(2) = \frac{5}{4} \end{align} \]

by first showing that the general solution is

\[y(x) = \frac{C}{x^2} + \frac{3}{4}.\]

What is the interval of existence for the solution to the IVP?

To see that \(y(x)\) is a general solution, you will need to find the derivative and plug it into the equation. The derivative is

\[ y'(x) = \frac{-2C}{x^3} .\]

Plugging that in, you get

\[ \begin{align} 2xy'+4y &= 2x\left(\frac{-2C}{x^3} \right) + 4\left(\frac{C}{x^2} + \frac{3}{4} \right) \\ &= \frac{-4C}{x^2} + \frac{4C}{x^2} + 4\left(\frac{3}{4} \right) \\ &= 3, \end{align}\]

which is the right hand side of the original differential equation. Therefore \(y(x)\) is a general solution.

Notice that the general solution is not defined at \(x = 0\). Therefore the interval of existence is either going to be \( (-\infty , 0)\) or \((0, \infty)\). Since the initial value is when \(x=2\), the interval of existence for the IVP is \((0, \infty)\).

Now to solve the IVP. Using the initial value,

\[ y(2) = \frac{C}{2^2} + \frac{3}{4} = \frac{5}{4}. \]

Solving for \(C\), you can see that \(C=2\). So the solution to the IVP is

\[y(x) = \frac{2}{x^2} + \frac{3}{4}.\]

Sometimes you will have a solution which is implicit, and that can be used to solve an IVP as well.

\[ y^2 = x^2 - 3\]

an implicit solution to the IVP

\[ \begin{align} &y' = \frac{x}{y} \\ &y(2) = 1 ?\end{align} \]

This requires the use of implicit differentiation. But before doing that, it is a good idea to make sure the proposed solution satisfies the initial value, because if it doesn't then it certainly can't be a solution to the IVP! Checking, the left hand side gives you \((y(2))^2 = 1^2 = 1\), and the right hand side gives you \(2^2 - 3 = 1\). Since they are the same, the proposed implicit solution at least satisfies the initial value.

Then using implicit differentiation and remembering that \(y\) is a function of \(x\),

\[ \frac{\mathrm{d}}{\mathrm{d} x} (y(x))^2 = \frac{\mathrm{d}}{\mathrm{d} x} \left(x^2 - 3 \right).\]

Looking first at the left hand side, and using the fact that if it is a solution then

\[y' = \frac{x}{y} ,\]

\[ \begin{align} \frac{\mathrm{d}}{\mathrm{d} x} (y(x))^2 &=2y(x)y'(x) \\ &= 2y\frac{x}{y} \\ &= 2x.\end{align}\]

On the right hand side,

\[ \frac{\mathrm{d}}{\mathrm{d} x} \left(x^2 - 3 \right) = 2x.\]

Since both sides are the same, \(y(x)\) does satisfy the differential equation.

Therefore the proposed implicit solution does actually solve the IVP.

What do you do if you can't find an implicit or explicit solution to an IVP?

Numerical Initial Value Problems in Ordinary Differential Equations

Most IVPs you have seen here you could find an explicit solution for, or at least an implicit one! What do you do if you have an IVP that you can't solve like that? That is when you need to use numerical methods to find solutions to IVPs. The method that most people start with is Euler's Method . It uses an initial value, and the differential equation, to calculate slopes that lead to an approximation of the solution to an ordinary differential equation IVP. For more information on this topic, along with plenty of examples, see Euler's Method.

Initial Value Problem Differential Equations - Key takeaways

Frequently Asked Questions about Initial Value Problem Differential Equations

--> how to solve initial value problems in differential equations .

First find the general solution to the differential equation.  Then plug in the initial value to find the constant of integration from the general solution.

--> What is initial value problem in differential equation? 

It is a differential equation where the solution is required to go through a specific point.

--> What is a initial value example? 

It is the point the solution to the differential equation needs to go through.  For example y' = 2x + 3 is a differential equation, and y(0) = 2 would be an initial value.

--> What is the initial value in an equation? 

--> does the initial value problem have a unique solution .

It depends on the differential equation.  If it is a first order linear one where the coefficients are continuous functions then the initial value problem has a unique solution.  

Final Initial Value Problem Differential Equations Quiz

What is the abbreviation for an initial value problem in differential equations?

Show answer

Show question

In terms of the solution of a differential equation, what does the initial value tell you?

It gives you a point the solution must go through.

What is the notation for an IVP?

\[ \begin{align} y' &= f(x,y) \\ y(a) &= b \end{align} \] 

What does a first order linear constant coefficient IVP look like?

\[ \begin{align} y'+Ay&=B \\ y(x_1) &= y_1, \end{align}\]

where \(A, B, x_1\), and \(y_1\) are real numbers with \(A \neq 0\).

What is the solution to 

\[ \begin{align} y(x) &= \frac{y_1 - \frac{B}{A}}{e^{-Ax_1}} e^{-Ax}+\frac{B}{A} \\ &= \left(y_1 - \frac{B}{A} \right)e^{-A(x-x_1)} +\frac{B}{A}  . \end{align}\]

Name 4 properties of first order linear constant coefficient IVPs.

Solutions to the IVP are unique .  

If you have two different initial values, the solutions corresponding to those initial values cannot cross.  

Solutions all have the same long term behavior.  

For what kind of differential equation are you guaranteed a unique solution that exists on the whole real line?

First order linear constant coefficient initial value problems.

When does a first order linear IVP have a unique solution on the whole real line?

If \(a, b \in \mathbb{R}\), and \(P(x)\), \(Q(x)\) are both continuous functions on the whole real line then the solution to the initial value problem 

\[\begin{align} y' + P(x)y &= Q(x) \\ y(a) &= b \end{align}\]

What does a separable differential equation IVP look like?

So for real numbers \(a\) and \(b\), the IVP is 

\[\begin{align} N(y)y' &= M(x) \\ y(a) &= b. \end{align}\]

What do you have to look out for when solving separable IVPs?

You need to be sure to pick the interval of existence that contains the initial value.

What can you do if you can't find an explicit or implicit solution to an IVP?

Try a numerical method, like Euler's Method.

What is Euler's Method used for?

Finding a numerical approximation to the solution of an IVP.

Do all IVPs have a solution?

No.  Some don't have solutions at all.

Do IVPs all have unique solutions?

No, some IVPs have two, or sometimes even more, solutions!

Will you always be able to find an explicit solution to an IVP, assuming there is a solution?

No.  Sometimes all you will be able to find is an implicit solution.

of the users don't pass the Initial Value Problem Differential Equations quiz! Will you pass the quiz?

More explanations about Calculus

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Introduction to Differential Equations

Calculus - introduction to differential equations and solved problems - outline of contents:, target audience: high school students, college freshmen and sophomores, students preparing for the  international baccalaureate (ib), ap calculus ab, ap calculus bc, a level, singapore/gce a-level;  class 11/12 students in india preparing for isc/cbse and entrance examinations like the iit-jee/aieee anyone else who needs this tutorial as a reference, differential equations, here's a quick outline of what this tutorial introduces :, • differential equation: , • ordinary differential equation: , • partial differential equation: , • order and degree of a differential equation: , • linear and non-linear differential equations: , • general,particular and singular solutions:  , • initial-value problem(ivp) and boundary-value problem(bvp): , • linear independence and dependence:  , • homogeneous equations: , • first order linear differential equations, • characteristic/auxiliary equation: , in case you'd like to take a look at some of our other tutorials related to single variable calculus :, our calculus tutorials                 .

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Math Insight

Examples of solving linear ordinary differential equations using an integrating factor.

Solve the ODE \begin{gather} \diff{x}{t} -\cos(t)x(t)=\cos(t) \label{example1} \end{gather} for the initial conditions $x(0)=0$.

Solution : Since this is a first order linear ODE, we can solve it by finding an integrating factor $\mu(t)$. If we choose $\mu(t)$ to be \begin{align*} \mu(t) = e^{-\int \cos(t)} = e^{-\sin(t)}, \end{align*} and multiply both sides of the ODE by $\mu$, we can rewrite the ODE as \begin{align*} \diff{}{t}(e^{-\sin(t)}x(t)) &= e^{-\sin(t)}\cos(t). \end{align*} Integrating with respect to $t$, we obtain \begin{align*} e^{-\sin(t)}x(t) &= \int e^{-\sin(t)}\cos(t) dt +C\\ &= -e^{-\sin(t)} + C, \end{align*} where we used the $u$-subtitution $u=\sin(t)$ to compute the integral. Dividing through by $e^{-\sin(t)}$, we calculate that the the general form of the solution to equation \eqref{example1} is \begin{gather*} x(t) = -1 + Ce^{\sin(t)}. \end{gather*}

We verify that we have a solution to equation \eqref{example1}. Since $$\diff{x}{t} = Ce^{\sin(t)}\cos(t)$$ we calculate that $$\diff{x}{t} - \cos(t)x(t) = Ce^{\sin(t)}\cos(t)-\cos(t)(-1 + Ce^{\sin(t)}) = \cos(t),$$ demonstrating that we have found the general solution to the ODE.

We determine the integration constant $C$ by the condition $0=x(0)=-1+Ce^0 = -1+C$, so that $C=1$. Our specific solution to the ODE of \eqref{example1} is \begin{gather*} x(t) = -1 + e^{\sin(t)}. \end{gather*}

Solve the ODE \begin{gather} \diff{x}{t} = \frac{1}{\tau}(-x + I(t)) \label{example2} \end{gather} with initial condition $x(t_0)=x_0$.

Solution : Rewrite the equation in the form \begin{gather*} \diff{x}{t} + \frac{x}{\tau}= \frac{I(t)}{\tau}. \end{gather*} In this case, our integrating factor is $\mu(t) = e^{\int (1/\tau) dt} = e^{t/\tau}$. Multiplying through by $\mu(t)$, we can rewrite our ODE as \begin{gather*} \diff{}{t}\left(e^{t/\tau}x(t)\right)= \frac{I(t)}{\tau}e^{t/\tau}. \end{gather*}

For this example, let's integrate from $t_0$ to $t$, rather than calculate the indefinite integral as in previous examples. \begin{align*} \int_{t_0}^t \diff{}{t'}\left(e^{t'/\tau}x(t')\right)dt' &= \int_{t_0}^t \frac{I(t')}{\tau}e^{t'/\tau}dt'\\ e^{t/\tau}x(t) - e^{t_0/\tau}x(t_0) &= \frac{1}{\tau}\int_{t_0}^t e^{t'/\tau}I(t')dt' \end{align*} Dividing through by $e^{t/\tau}$ and using the initial conditions $x(t_0)=x_0$, the solution to the ODE of \eqref{example2} is \begin{gather} x(t) = e^{-(t-t_0)/\tau}x_0 + \frac{1}{\tau}\int_{t_0}^t e^{-(t-t')/\tau}I(t')dt'. \label{example2sol} \end{gather}

To verify this solution, we differentiate equation \eqref{example2sol} with respect to $t$, obtaining three terms (two from the exponentials and one from the upper integration limit), \begin{align*} \diff{x}{t} &= -\frac{1}{\tau} e^{-(t-t_0)/\tau}x_0 -\frac{1}{\tau} \frac{1}{\tau}\int_{t_0}^t e^{-(t-t')/\tau}I(t')dt' + \frac{1}{\tau} e^{-0/\tau}I(t) \\ &= -\frac{1}{\tau} \left(e^{-(t-t_0)/\tau}x_0 +\frac{1}{\tau}\int_{t_0}^t e^{-(t-t')/\tau}I(t')dt'\right) + \frac{1}{\tau} I(t) \\ &=-\frac{1}{\tau}x(t) + \frac{1}{\tau}I(t). \end{align*} Indeed $x(t)$ satisfies equation \eqref{example2}. If we plug $t=t_0$ into equation \eqref{example2sol}, the integral is from $t_0$ to $t_0$, so is zero. The exponential of the first term is $e^0=1$, and we confirm that $x(t_0)=x_0$.

Solve the ODE \begin{align} \diff{x}{t} + e^{t}x(t) = t^2 \cos(t) \label{example3} \end{align} with the initial condition $x(0)=5$.

Solution : The first step is to find the integrating factor \begin{gather*} \mu(t) = e^{\int e^t dt} = e^{e^t} = \exp(e^t), \end{gather*} where $\exp(x)$ is another way of writing $e^x$. Multiplying equation \eqref{example3} by $\mu(t)$, then the left hand side is the derivative of $\mu(t)x(t)$. We can write it as \begin{align*} \diff{}{t}\left( \exp(e^t) x(t)\right) = t^2 \cos(t) \exp(e^t). \end{align*}

To solve the ODE in terms of the initial conditions $x(0)$, we integrate from $0$ to $t$, obtaining \begin{align*} \int_0^t \diff{}{s}\left( \exp(e^{s}) x(s)\right)ds &= \int_0^t {s}^2 \cos(s) \exp(e^{s})ds\\ \exp(e^{t}) x(t) - \exp(e^{0}) x(0) &= \int_0^t {s}^2 \cos(s) \exp(e^{s})ds. \end{align*} Even though we cannot compute the integral analytically, we still consider the ODE solved. Plugging in the initial conditions $x(0)=5$, we can write the solution of the ODE \eqref{example3} as \begin{align*} x(t) &= 5\exp(1-e^t)+ \int_0^t {s}^2 \cos(s) \exp(e^{s}-e^{t})ds. \end{align*}

You can easily check that $x(t)$ satisfies the ODE \eqref{example3} and the initial conditions $x(0)=5$.

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IMAGES

  1. Linear Differential Equation: Properties, Solving Methods, Videos, Example

    example problem in linear differential equation

  2. Linear Differential Equations

    example problem in linear differential equation

  3. IVP: First Order Linear Differential Equation: Example 2

    example problem in linear differential equation

  4. Diff Eq: Solving Non-Exact differential equations: Example 2/5

    example problem in linear differential equation

  5. IVP: First Order Linear Differential Equation: Example 1

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  6. Linear Differential Equations. Introduction and Example 1.

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  1. A01 Introduction to linear systems

  2. Differential Equations: Linear

  3. DE 41 Special Second Order Differential Equations

  4. Solve the differential equation

  5. DE 42 Special Second Order Differential Equations _ Example #5

  6. DIFFERENTIAL EQUATION

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  1. 17.1: Second-Order Linear Equations

    Consider the differential equation x ″ + 7x ′ + 12x = 0. Both e − 3t and 2e − 3t are solutions (you can check this). However, x(t) = c1e − 3t + c2(2e − 3t) is not the general solution. This expression does not account for all solutions to the differential equation.

  2. Differential Equations (Practice Problems)

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  3. Linear Differential Equation (Solution & Solved Examples)

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  4. Differential Equations

    The solution to a linear first order differential equation is then y(t) = ∫ μ(t)g(t) dt +c μ(t) (9) (9) y ( t) = ∫ μ ( t) g ( t) d t + c μ ( t) where, μ(t) = e∫p(t)dt (10) (10) μ ( t) = e ∫ p ( t) d t Now, the reality is that (9) (9) is not as useful as it may seem.

  5. Linear Differential Equation

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  6. Linear vs nonlinear differential equation

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  7. Linear differential equation problems and answers

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  8. Linear Differential Equation

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  9. Linear differential equations of first order: Solved Example Problems

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  10. Worked example: linear solution to differential equation

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  11. Differential Equations

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  12. PDF LINEAR DIFFERENTIAL EQUATIONS

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  16. Linear differential equation

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  17. Calculus

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  18. Examples of solving linear ordinary differential equations using an

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  19. Examples of a nonlinear differential equation

    What is an nonlinear ordinary differential equation and give an. An equation in which the maximum degree of a term is 2 or more than two is called a nonlinear equation. + 2x + 1 = 0, 3x + 4y = 5, this is the example of. order now.

  20. Nonhomogeneous differential equation examples

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  21. Differential equation of a polynomial function

    Order and Degree of Differential Equations with Examples. Linear differential equations. The general linear ODE of order n is. (1) think of the formal polynomial p(D) as operating on a function y(x), converting. ... Math is a way of solving problems by using numbers and equations.

  22. Ordinary differential equations solving methods

    Ordinary differential equation examples. by GE Sjoden Cited by 6 - An Ordinary Differential Equation (ODE) is an equation that defines a relationship between an independent variable x and a dependent variable y ... Inspection Method Variable Separable Method Homogenous Differential Equations Linear Differential Equation. ... To determine what ...

  23. Solving system of differential equations with initial conditions

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  24. Non homogeneous partial differential equations examples

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