1. The problem statement, all variables and given/known data 3. The attempt at a solution I really need some help here. I've been able to get through about 50 pages of calculus without really understanding what it means to differentiate both sides with respect to t, now I've run out of space to hide and I can't succeed any more without understanding this concept. I don't see why here differentiating both sides with respect to t forces me to take the inverse of (pi(h^2))/4. any help would be really appreciated. If you want me to differentiate (pi(h^2))/4, then to me is simply h, because pi as a number turns to zero, 4 turns to zero and h squared turns to h.