# Homework Help: Proving a differential equation using the substitution method

1. Oct 2, 2008

### jeffreylze

1. The problem statement, all variables and given/known data

dy/dx = $$\frac{[2cos(x)^2-sin(x)^2+y^2]}{[2cos(x)]}$$

Substitute y(x) = sin(x) + $$\frac{1}{u(x)}$$
2. Relevant equations

By doing the substitution, it will yield the differential equation for u(x)

du/dx = -u tan(x) - $$\frac{1}{2}$$sec(x)

3. The attempt at a solution

I figured out i have to use chain rule. However, if du/dx = du/dy x dy/dx , which dy/dx do i choose? It can be either

this - dy/dx = $$\frac{[2cos(x)^2-sin(x)^2+y^2]}{[2cos(x)]}$$

or - y = sin(x) + 1/[u(x)]
dy/dx = cos(x)

Then, I found the dy/du from this equation y = sin(x) + 1/[u(x)] and flipped it around to get du/dy. After multiplying using the chain rule, I dont get the differential equation as shown. Please help me out here. =X

2. Oct 2, 2008

### jeffreylze

I just realized a similar question has been posted. Please close the thread. Sorry for the trouble. =X