# Homework Help: Differential Equation ?

1. Apr 19, 2008

### thorjj

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

Solve (x^2 + y^2 - y)dx + x dy = 0 when x > 0, y > 0.

2. Relevant equations

(x^2 + y^2 - y)dx + x dy = 0

3. The attempt at a solution

dy/dx = - (x^2 + y^2 - y) / x
That seems like a tricky one. Can I make it a second-order diff. eq.?
Well, with the topic we have had about vectorfields I can backwards engineer it into
F(x,y) = (x^2 + y^2 - y)i + (-x)j
But that dosn't make much sense to me.

2. Apr 19, 2008

### thorjj

(Hmm, I miss an edit-button...)

I shorted it down to x dy/dx = -x^2 - y^2 + y or dy/dx = -x -y^2/x + y/x

3. Apr 19, 2008

### Defennder

You don't have to complicate this one unnecessarily. The substitution y=vx works fine here.

4. Apr 19, 2008

### Shooting Star

$$(x^2+y^2)dx + (xdy-ydx) = 0 => dx + \frac{xdy-ydx}{x^2+y^2} = 0.$$

Whenever you get (xdy-ydx), see if you can make it into the form

$$\frac{xdy-ydx}{x^2}$$, since that is equal to $$d(\frac{y}{x}).$$

See if you can spot a function of (y/x) with the d(y/x) here.