Originally Posted by

**Mush** The directional derivative is:

$\displaystyle D_v f = \frac{\vec{v} . \nabla f}{|\vec{v}|} $

f is your function, and v is the direction in which you are calculating the derivative.

The direction in which the line is steepest is when $\displaystyle v = \nabla f $

Hence the directional derivative in that direction is:

$\displaystyle D_v f = \frac{\nabla f . \nabla f}{|\nabla f|} = \frac{|\nabla f|^2}{|\nabla f|} = |\nabla f| $

So

1) Find $\displaystyle \nabla f$ in terms of x, y and z.

2) Plug in your point (240,80,168)

3) Find the magnitude of the vector you get.