# Homework Help: Gradient vector problem

1. Sep 5, 2011

### evo_vil

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

If $z = f(x,y)$ such that $x = r + t$ and $y = e^{rt}$, then determine $\nabla f(r,t)$

2. Relevant equations

$\nabla f(x,y) = <f_x,f_y>$

3. The attempt at a solution

Now if i follow this the way i think it should be done then i find the partials of f wrt x and y and then simply sub in r and t in the place of x and y respectively...

But if i get del f the normal way i get:

$\nabla f = <f_x+f_y t e^{rt},f_x+f_y r e^{rt}>$

is this the final/correct answer or am i missing a trick question where i was asked to find del f(r,t) and not del f(x,y)

2. Sep 5, 2011

### I like Serena

Welcome to FP, evo_vil!

If I take it very literal, the answer would be:
$$\nabla f(r,t)=<f_x(r,t), f_y(r,t)>$$

However, I can't imagine that this was intended.

I expect that you're supposed to take the gradient from a function f* defined by:
f*(r,t) = f(x(r,t), y(r,t)).
It is not unusual that this function f* is simply called f, although that is ambiguous.

This is what you calculated, and no doubt correct.

3. Sep 5, 2011

### evo_vil

Thanks!

Ive browsed PF for quite a few years, but never participated, so thanks for the welcome...

I think im just going to go with what ive calculated and see how it goes...
Maybe see if other people get the same thing.