Consider the function $f(x,y,z)=xy+y{z}^2+x{z}^3.$ Find the gradient $off$ :

$$

Find the gradient $offat$ the point $(2,1,2).$

Find the rate

$of$ change $of$ the function $fat$ the point $(2,1,2)in$ the direction

$u=($:$-\frac{3}{\sqrt[\phantom{2}]{59}},\frac{5}{\sqrt[\phantom{2}]{59}},-\frac{5}{\sqrt[\phantom{2}]{59}}$:$).$