# I Derivative When Substituting Variables

1. Jun 18, 2017

### Staff: Mentor

I'm working through a proof in my differential equations book, but I think I'm hung up on a basic calculus derivative.

If we have a function $f(x,y)$ and we substitute $v=\frac{y}{x}$ , rearrange to get $y=vx$, and then take the derivative, supposedly by the product rule we get $$\frac{dy}{dx}=v+x\frac{dv}{dx}$$
I'm not quite sure how this works since $v$ is a function of both y and x and y itself is a function of x. What's going on here?

Last edited: Jun 19, 2017
2. Jun 18, 2017

### cnh1995

I think it should be dy/dx=v+x*dv/dx as per the product rule.

3. Jun 19, 2017

### Staff: Mentor

I'm sorry, I forgot to add the X in the 2nd term. My mistake.
I'm still not sure what's going on though.

4. Jun 19, 2017

### Staff: Mentor

$v = \frac y x$, so in a sense v is a function of both x and y, but the assumption is that y is a function of x. This means that v is also a function of x alone.

Starting with the equation y = vx, differentiate both sides with respect to x. This gives you y' = v + v'x, just using the product rule.

5. Jun 19, 2017

### Staff: Mentor

Okay. I figured it was something easy I was missing. Thanks guys.