- #1
ynotidas
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Homework Statement
Solve the nonlinear ODE
du/dx=(u+x√(x^2+u^2 ))/(x-u√(x^2+u^2 ))
by changing variables to x=rcos(theta), u=rsin(theta) and converting the equation to one for d(theta)/dr.
The Attempt at a Solution
Not sure if I'm going in the right direction.
du/dx = du/d(theta) x d(theta)/dx
u = rsin(theta), du/d(theta) = rcos(theta)
x = rcos(theta), dx/d(theta) = -rsin(theta),
i.e. d(theta)/dx = -1/rsin(theta)
so du/dx = rcos(theta)/-1rsin(theta) = - cos(theta)/sin(theta)
then i sub x=rcos(theta), u=rsin(theta) in the main equation
- cos(theta)/sin(theta) = rsin(theta)+rcos(theta)√(r^2sin^2(theta) etc etc..
so i gather the r^2, and make the sin^2+cos^2 both to one, then the √r^2 goes to just 'r'.
then i take divide the whole equation by r
du/dx = -cos(theta)/sin(theta) = [sin(theta)+rcos(theta)]/[rcos(theta)-rsin(theta)
so i times the sin(theta) over the right side and the other to the right..
and i get 1=0...
can someone put me in the right direction on what i did wrong?
thankyou.