# Differential Eqns Separable Equation?

1. Jul 30, 2008

### Prologue

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

Problem given basically in the beginning of the book, Sec. 1.4, about separable equations.

Find general solutions (implicit if necessary, explicit if convenient) of the diff. eq.

$$dy/dx+2xy^2=0$$

2. Relevant equations

There was a previous section where they popped this out of thin air:
A solution of $$dy/dx=y^2$$ is $$y=1/c-x$$

I'm presuming that that is supposed to be helpful somehow.

3. The attempt at a solution

$$dy/dx=-2xy^2 => dy/y^2=-2xdx$$

Now I can't integrate the left side.

I'm sure that is an abysmal way of going at it. So any help would be appreciated.

2. Jul 30, 2008

### HallsofIvy

Staff Emeritus
That's a perfectly good way of going at it. The question is why are you having trouble integrating y-2dy? Didn't you learn a general formula for integrating powers? (Other than -1)

3. Jul 30, 2008

### Prologue

I haven't learned a formula for that. I will bet that it involves grabbing the derivative of the denominator from somewhere and thinking of it as a u-substitution but I'm not seeing where that is possible.

EDIT: Ok I just ignored that y was a function of x and integrated, and it worked out perfectly fine. I am supposing that just this idea is a point of this exercise. When you integrate something in this way, you have an equation and integrate both sides. When you do it that way y is no longer taken as being a function of x. I can accept that as the point, but now why is that possible?

Last edited: Jul 30, 2008
4. Jul 30, 2008

### konthelion

Ah, I think you're looking at it like this $$\frac{f'(x)}{[f(x)]^2}$$ in your head, and somehow you think of it as a u-substitution where u = f(x).

Basically the general formula for integrating is

$$\int{x^rdx}= \frac{1}{r+1} x^{r+1}+C$$ where $$r \neq -1$$

5. Jul 30, 2008

### Prologue

Thanks for the reply, konthelion. I was in the process of editing the last post when you posted, I apologize.

I do know that general rule of integration (it has been severely beaten into my brain) but I was having trouble with the y. My head is saying 'Hey, hold on a minute y is a function of x, you can't do that.' Apparently my head is wrong.