# Math Help - scalar/vector field question

1. ## scalar/vector field question

hey guys, I've been asked to consider the scalar field:

f(x,y,z) = xy + xz + yz

After finding grad f, I am asked to find the directional derivative of f at the point P (1,1,1) along the vector field u = [2,-1,1].

What confuses me is finding the directional derivative of f along the vector field u... do i just calculate the dot product of grad f and the unit vector of u?

grad f = (y+z)i + (x+z)j + (x+y)k

The answer i got when performing the calculations the way I believe to be correct is $2\sqrt{6}/3$, does anybody know if this is correct?