1. The problem statement, all variables and given/known data Here is the problem with the solution: http://dl.dropbox.com/u/64325990/MATH%20253/Capture.PNG [Broken] What I don't understand is circled in red, how did they combine dxdy with dydx? Is it with Clairaut's theorem? If it is can someone explain how it works in this case because the second derivative makes this very confusing. Additionally 4s^2 ≠ 4r^2 so how can you use Clairaut?