In Exercises 13 through 24 , find the indicated partial derivatives by holding all but one of the variables constant and applying theorems for ordinary differentiation.
$$
u=\left(x^{2}+y^{2}+z^{2}\right)^{-1 / 2} ; \frac{\partial u}{\partial z}
$$
20. $u=\tan ^{-1}(x y z w) ; \frac{\partial u}{\partial w}$