# Homework Help: Directional derivatives and partial derivatives

1. Nov 12, 2012

### The1TL

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

Suppose f: R -> R is differentiable and let h(x,y) = f(√(x^2 + y^2)) for x ≠ 0. Letting r = √(x^2 + y^2), show that:

x(dh/dx) + y(dh/dy) = rf'(r)

2. Relevant equations

3. The attempt at a solution
I have begun by showing that rf'(r) = sqrt(x^2 + y^2) * limt->0 (f(r+t) - f(r))/t

and written out the definition form of the directional derivatives. I cant seem to find a way to equate both sides of the equation. Can anyone help?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 12, 2012

### HallsofIvy

I don't see that there is any "directional derivative" involved here. You are given that $h(x,y)= f(\sqrt{x^2+ y^2})$ . By the chain rule $\partial f/\partial y= (df/dr)(\partial r/\partial y)$ and $\partial f/\partial x= (df/dr)(\partial r/\partial x$.

With $r= \sqrt{x^2+ y^2}$, it is easy to find $\partial r/\partial x$ and $\partial r/\partial y$.