multivariable calculus

Let w = f (x, y), where x and y are functions of t. Then, according to the chain rule, dw/dt is equal to

If $$f\left( {x,y,z} \right) = $$ $${\left( {{x^2} + {y^2} + {z^2}} \right)^{{\raise0.5ex\hbox{$\scriptstyle { - 1}$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 2$}}}...