# Differential Equations

• Sep 29th 2006, 11:18 PM
primasapere
Differential Equations
There's this problem that's asking me to use Euler's equidimensional equation, which is a second order differential equation. The problem tells me to substitute x = exp(z) to transform the equation into one with constant coefficients, and apply the technique to various examples.

Just to illustrate, Euler's equidimensional equation looks like this:

(x^2)*y'' + p*x*y' + q*y = 0, where p and q are constants. One of my tasks is to use the substitution x = exp(z) to solve the following problem:

(x^2)*y'' + 3*x*y' + 10*y = 0

Anyone have an idea how to use the substitution to my advantage? Just inserting exp(z) for x doesn't work. Should I be changing y' to be in terms of z?

Any help appreciated, thanks.
• Sep 30th 2006, 01:29 AM
CaptainBlack
Quote:

Originally Posted by primasapere
There's this problem that's asking me to use Euler's equidimensional equation, which is a second order differential equation. The problem tells me to substitute x = exp(z) to transform the equation into one with constant coefficients, and apply the technique to various examples.

Just to illustrate, Euler's equidimensional equation looks like this:

(x^2)*y'' + p*x*y' + q*y = 0, where p and q are constants. One of my tasks is to use the substitution x = exp(z) to solve the following problem:

(x^2)*y'' + 3*x*y' + 10*y = 0

Anyone have an idea how to use the substitution to my advantage? Just inserting exp(z) for x doesn't work. Should I be changing y' to be in terms of z?

Any help appreciated, thanks.

Put x = exp(z), then:

z = ln(x)

so:

dz/dx = 1/x = exp(-z).

Now:

y' = dy/dx = dy/dz dz/dx= exp(-z) dy/dz

and:

y'' = d^2y/dz^2 =d/dz[exp(-z) dy/dz)] dz/dx

....= [-exp(-z)dy/dz + exp(-z) d^2/dz^2] exp(-z)

....= exp(-2z) d^2y/dz^2 - exp(-2z) dy/dz.

Substituting these expressioons for x, y', and y'' into
x^2 y'' + 3 x y' + 10 = 0, gives:

d^2y/dz^2 -dy/dz + 3 dy/dz + 10 = 0,

or:

d^2y/dz^2 + 2 dy/dz + 10 = 0.

RonL
• Sep 30th 2006, 09:48 AM
primasapere
Wow. That sort of approach was completely over my head. Thanks a lot!