If $r=+9y+$hat$(z)z$ and $r=|r|,$ prove the following identity

grad$(\frac{1}{r})=-\frac{r}{r^3}$

$4$

$5.$ Given the vector field $A=$hat$(x)(3x^3-4z^2)+$hat$(y)6x^{2}y-$hat$(z)5x^2,$ evaluate the volume integral ${}_V$grad$*AdV,$ where $V$ is the region bounded by the planes $x=0,y=0,z=0,$ and $2x+2y+z=4.$