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Trouble with a second order differential equation

  • Thread starter draco193
  • Start date
  • #1
7
0
Hello all,

I'm having a little trouble getting the Latex to work, so I'm hoping this wont be too hard for everyone to understand.

Homework Statement



I am given the second order differential equation
x2*y''(x)+(2*b+1)*x*y'(x)+c*y(x) = 0

Use the transform x=ez to find the general solution.

b,c are real, and b2>c

Then, using the special solution y=xp, solve for p and confirm you get the same solution.

The find the real solutions if b2=c, b2<c

Homework Equations



The Quadratic Formula will be used


The Attempt at a Solution



For the first part, transforming the original equation:

y'(x) = [tex]\frac{dy}{dz}[/tex]*[tex]\frac{dz}{dx}[/tex] =y'(z)*[tex]\frac{1}{x}[/tex]

similarly, y''(x) = [tex]\frac{-1}{x^2}[/tex].*y''(z)

Replacing into the original equation, I get
-y''(z)+(2b+1)*y'(z)+c*y(z).

Using the quadrtaic equation, I then get as a solution
[tex]\frac{(2b+1) \mp sqrt((4b^2)+4b+4c+1}{2}[/tex]

Using these as r1 and r2, and replacing z with x, I find the general solution y(x) = c*xr1 + c*xr2

So then solving the special solution

x2*(p-1)*(p)*xp-2+x*p*xp-1+c*xp = 0

Simplifying, I get

p2+2bp+c =0

Solving for p, I get p = -b[tex]\pm[/tex] sqrt(b2-c)

This obviously does not equal my other solution, so I'm wondering if anyone can see where it is I've made my mistake?
 
Last edited:

Answers and Replies

  • #2
95
0
You need to calculate y''(x) not y''(z).
 
  • #3
7
0
You need to calculate y''(x) not y''(z).
Thats what I get for trying to type my work in quickly. Typo fixed.
 
  • #4
95
0
I got something different for y''(x). Did you calculate

[tex]\frac{d^{2}y}{d^{2}x}=\frac{d(\frac{dy}{dx})}{dz}*\frac{dz}{dx}[/tex]

using the product rule for derivatives on

[tex]\frac{d(\frac{dy}{dx})}{dz}[/tex]

?
 
  • #5
7
0
I got something different for y''(x). Did you calculate

[tex]\frac{d^{2}y}{d^{2}x}=\frac{d(\frac{dy}{dx})}{dz}*\frac{dz}{dx}[/tex]

using the product rule for derivatives on

[tex]\frac{d(\frac{dy}{dx})}{dz}[/tex]

?
Thank you. That is where my error was at.
 

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