# Homework Help: Strange ODE from my final today?

1. Dec 6, 2011

### Agent M27

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

So I had my final exam today in ODE and I had an equation which appeared to be exact, but was not. I also tried to find a special integrating factor to make it exact, but no success. I then attempted to manipulate it into a linear eq, tried separable variables, even tried to get it into a Bernoulli form, all of which no luck. This was the extra credit question so I know it must be difficult, but I cannot find an example similar to it in our text, "Fundamentals of Differential Equations" by Nagle. Here is the equation and I was told I could use any method I like to solve it.

x(x-y-2)dx+y(y-x+4)=0

2. Relevant equations

It is not an exact differential, cannot obtain a special integrating factor. Thanks again for any assistance, this problem has been bugging me all day!

3. The attempt at a solution

2. Dec 7, 2011

### Staff: Mentor

There should be a dy somewhere in this problem.

3. Dec 7, 2011

### Agent M27

Crap you're correct Mark, the equation actually read as x(x-y-2)dx+y(y-x+4)dy=0. I have determined that I need to do some sort of substitution, using v=$y/x$, which ought to transform it into some sort of separable variables. However, I am still running into trouble attempting to separate into dv=dx. Here is where I get stuck:

$\frac{dy}{dx}$=-$\frac{x(x-y-2)}{y(y-x+4)}$

dividing by x

=-$\frac{(1-y/x-2/x)}{y/x(y/x-1+4/x)}$=-$\frac{(x-y-2)}{(y^{2}/x-y+4y/x)}$

substituting $v$=$y/x$, $y=vx$, $\frac{dy}{dx}$=$\frac{dv}{dx}x$+$v$ and some rearranging and I get stuck...

$\Rightarrow$ $\frac{dv}{dx}$ = -$\frac{(x-vx-2)}{(v^{2}x-vx^{2}+4vx)}$-$\frac{v}{x}$

Do I now need to come up with another substitution? Thanks for you help.

Joe