# Homework Help: Directional Derivative question. Calculus III/2-variable.

1. Oct 31, 2012

### Unart

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

Let f(x,y,z)=sin(xy+z) and P=(0,-1, pi), Calculate Duf(P) where u is a unit vector making an angle 30°
=θ with dFp?
2. Relevant equations
This is a two variable problem, where partials are necessary in order to find the gradient formula to get <dFP>. To take the Directional Derivative, from what I know, you require the following equation... <dFP>dot<U>/||u||.

3. The attempt at a solution
from the following I calculated the gradient equation to be
<ycos(xy+z),xcos(xy+z),cos(xy+z)>

with the given gradient equation, I get dFP as <1,0,-1>.

What I cannot figure out is how to get the coordinates for the U vector.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution