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rock.freak667
Nov14-08, 11:08 PM
1. The problem statement, all variables and given/known data

Solve : y''+(2/x)y'+y=0 given that y=sinx/x is a solution

2. Relevant equations



3. The attempt at a solution
y=vsinx/x is the other solution

I worked out

y'=\frac{vxcosx-vsinx+v'xsinx}{x^3}

to work out y'' got extremely confusing for me,so I used an online differentiator to do it.
It gave y'' as

\frac{x[2(cosx-sinx)v'+v''xsinx]-v[2xcosx+(x^2-2)sinx]}{x^3}

Now when I substitute it back into the equation, I keep getting the terms for v to not cancel and I get one complex differential equation.

HallsofIvy
Nov15-08, 06:13 AM
Your first derivative is incorrect. The denominator should be x2, not x3.

rock.freak667
Nov15-08, 01:25 PM
I will check it over, is there any site where I can input all my differential equations that I need to solve and I can check if I have the correct answer?

Office_Shredder
Nov15-08, 01:28 PM
If you have a possible solution, just plug it into your differential equation and see if it works

rock.freak667
Nov15-08, 01:32 PM
If you have a possible solution, just plug it into your differential equation and see if it works

I can do that but I am afraid that I might make a mistake in differentiating the solution and it won't work with the equation and I'll think I did it wrong.

rock.freak667
Nov15-08, 02:03 PM
Ok here is my attempt (again)

y=vsinx/x

y'=(\frac{sinx}{x})v'+v(\frac{cosx}{x}-\frac{sinx}{x^2})

y''=v''(\frac{sinx}{x}+\frac{cosx}{x}-\frac{sinx}{x^2})+v'(\frac{cosx}{x}-\frac{sinx}{x^2})+v(-\frac{sinx}{x}-\frac{cosx}{x^2}-\frac{cosx}{x^3}+\frac{2sinx}{x^3}

I then put that into the equation and got

v''(\frac{sinx}{x}+\frac{cosx}{x}-\frac{sinx}{x^2})+v'(\frac{2cosx}{x})=0

Then let w=v' => w'=v''

thus getting
w'(\frac{sinx}{x}+\frac{cosx}{x}-\frac{sinx}{x^2})+w(\frac{2cosx}{x})=0

\times x^2

w'(xsinx+xcosx-sinx)+(2xcosx)w=0

Now

\int \frac{1}{w}dw= \int \frac{-2xcosx}{xsinx+xcosx-sinx}dx

and I have no idea how to integrate the term on the right side, I've tried substitutions but it didn't work out well.

EDIT: my differentiating over complicated the integral...I got it out now.