# How to Solve Linear Differential Equations

By Prashant Kumar|Updated : August 19th, 2018

Form of linear differential equation is Here P(x) and Q(x) are the function of x.

To solve this type of differential equation we use the integration factor. We don’t need the derivation on how we got the integration factor. Integration factor is given by Now both sides of the equation with IF and simplify: So the Final Form that we have to solve is Now we have to use simple integration concept and find the answer.

Example: Solution: Simplify this equation to get We can see this is linear differential equation. So we find the integration factor first. So the solution of this equation is given by Here C is the integral constant.

## DIFFERENTIAL EQUATION REDUCIBLE TO THE LINEAR FORM

We see how to solve a linear differential equation. Now here we see a different type of equation and that can be reducible to Linear differential equation form. To solve this, we use substitution method and convert this into linear form. So the equation is And this is Linear differential equation. Now we can solve this using linear differential method. Here we find the solution in u and x term. Then put back value of u = f(y).

Example: Solve Solution: -  We can write this equation as Now here we put New equation is And this is a linear differential equation. Find the IF of this equation So the solution of the equation is given by Put back the value of v ## EXACT DIFFERENTIAL EQUATION

The basic form of exact differential equation is

M(x, y) dx + N(x, y) dy = 0

Here M and N are function of x and y such that If this is exact differential equation then there exist a function f (x, y) such that And once we find that function then the general solution is

f(x, y) = c

So to find this we use know terms. and Finally, to find the solution we ‘merge’ both the functions obtained above. To do that, write down each term exactly once, even if a particular term appears in both results. And that merged form is called f(x, y) and the final solution is

f(x, y) = c

Example: Solution: Here we see that

M = 2xy - 9x2

N = 2y + x+ 1

And if we find Here we can see that

My = Nx

So this is an exact differential equation.

Now we find the solution. So for final answer we merge both solution we find. Here we can see that x2y is the common term in both solution so we write that only once.

f(x,y) = x2y - 3x+ y+ y

So the general solution is This is the final solution of the given exact differential equation. Here c is the integral constant.

## Solved Examples of Differential Equations

1. Solve the differential equation Solution:

1st we write this in separate from. Now integrate both side. So this is the final solution of differential equation.

1. Find the curve for which the intercept cut off by any tangent on y-axis is proportional to the square of the ordinate of the point of tangency.

Solution:

Let p(x, y) be any point on the curve

Length of intercept on y axis = y - x (dy/dx)

And by the question the length of intercept on y-axis is proportional to y2

So Here k is the constant of proportionality.

So the simple form of the equation is Put 1/y = v

So So the differential equation is And this is a linear differential equation So the solution is Now put back v=1/y

So So this is the final answer. Here C is the integral constant and k is the proportional constant.

1. Find the orthogonal trajectories of family of curves x2 + y2 = cx

Answer: So if 2 functions are orthogonal then the product of slope at any point curve is -1. Because they make 90-degree angle with each other. Now we find the slope (dy/dx) at any point (x,y) for the given curve and find the slope for the orthogonal curve and solve that. Now if the slope of other curve is M then So we take the slope of other curve as dy/dx, so the differential equation is And this is homogeneous equation so put

y =vx Here k = 1/C and it is a constant.

So the orthogonal curve to the given curve is  x+ y= ky

1. Solve Answer: We can write this as So here we can take (y+1)/x as a another variable and then solve it.

Put So the equation is How to Solve Homogeneous Differential Equation

How to Solve Differential Equations by Variable Separable Method

Order, Degree & Solutions of Differential Equations

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All the best!!

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