Determine the value of r in terms of l, k, and m for which the following function has a minimum. V(r) = -(k/r) + (l^2/(2mr^2)) where l, k, and m are positive constants. Prove that this is a minimum by showing that the second derivative of V(r) at the minimum is positive. I have no idea how to even begin this...I am horrible at derivatives and am struggling in my physics class with them. Any help would be greatly appreciated. I am then asked to derive Kepler's third law from Kepler's second law. So I feel I have a lot of work ahead of me.