1. The problem statement, all variables and given/known data -here is the problem statement -here is a bit of their answer 2. Relevant equations Chain rule, partial derivative in spherical coord. 3. The attempt at a solution I tried dragging out the constant and partial derivate with respect to t but still I can't reach their df/dt and d^2f/dt^2 partial derivatives. Any help would be much appreciated. Thank you !