What is the general solution of the differential equation x^2y''−xy'−3y=0?

emmostatwf

emmostatwf

Answered question

2022-09-28

What is the general solution of the differential equation x 2 y - x y - 3 y = 0 ?

Answer & Explanation

ralharn

ralharn

Beginner2022-09-29Added 15 answers

We have:
x 2 y - x y - 3 y = 0 ..... [A]
This is a Euler-Cauchy Equation which is typically solved via a change of variable. Consider the substitution:
x = e t x e - t = 1
Then we have,
d 2 y d x 2 = ( d 2 y d t 2 - d y d t ) e - 2 t
Substituting into the initial DE [A] we get:
x 2 ( d 2 y d t 2 - d y d t ) e - 2 t - x e - t d y d t - 3 y = 0
( d 2 y d t 2 - d y d t ) - d y d t - 3 y = 0
d 2 y d t 2 - 2 d y d t - 3 y = 0 ..... [B]
This is now a second order linear homogeneous Differentiation Equation. The standard approach is to look at the Auxiliary Equation, which is the quadratic equation with the coefficients of the derivatives, i.e.
m 2 - 2 m - 3 = 0
We can solve this quadratic equation, and we get two real and distinct solutions:
( m + 1 ) ( m - 3 ) = 0 m = - 1 , 3
Thus the Homogeneous equation [B]:
d 2 y d t 2 - 2 d y d t - 3 y = 0
has the solution:
y = A e - t + B e 3 t
Now we initially used a change of variable:
x = e t t = ln x
So restoring this change of variable we get:
y = A e - ln x + B e 3 ln x
y = A e ln x - 1 + B e ln x 3
y = A x - 1 + B x 3
y = A x + B x 3
Which is the General Solution

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