Chapter 12.6, Problem 21E

Computing directional derivatives with the gradient Compute the directional derivative of the following functions at the given point P in the direction of the given vector. Be sure to use a unit vector for the direction vector.

21     $f\left(x,y\right)=\sqrt{4-x^2-2y};P\left(2,-2\right);\langle \frac{1}{\sqrt{5}},\frac{2}{\sqrt{5}}\rangle$