yes the question did ask for ##\frac{d}{dt}f(x(t),y(t),z(t))## . the theoram used to solve is
##\frac{\partial f(x,y,z)}{\partial t}=\frac{\partial f(x,y,z)}{\partial x}\frac{\partial x}{\partial t}+\frac{\partial f(x,y,z)}{\partial y}\frac{\partial y}{\partial t}+\frac{\partial...