1. The problem statement, all variables and given/known data I'm trying to follow my text book on an application of the chain rule. Two objects are traveling in elliptical paths given by the following parametric equation. x1 = 4 cos t x2 = 2 sin 2t y1 = 2 sin t y2 = 3 cos 2t At what rate is the distance between the two objects changing when t = pi? 2. Relevant equations distance S = √(x2 - x1)2 + (y2 - y1)2 3. The attempt at a solution When t = pi x1 = -4 y1 = 0 x2 = 0 y2 = 3 When t = pi the partial derivatives of s are as follows. ∂s/∂x1 = -(x2 - x1)/√(x2 - x1)2 + (y2 - y1)2 = -1/5(0 + 4) = -4/5 How come we're all of a sudden dividing -(x2 - x1) by S? I guess I'm not grasping the concept of the partial derivative of S with respect to x1 when x1 is an equation either. Would someone be able to elaborate on that? If you need me to explain more, please let me know. Thanks in advance for any help.