1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution I have the solution to this problem and the issue I'm having is that I don't understand this step: Maybe I'm overlooking something simple but, for the red circled part, it seems to say that ∂/∂x(∂z/∂u) = (∂2z/∂u2)y+(∂2z/∂v∂u)(-y/x2). I realize that ∂2z/∂x∂u = ∂2z/∂u∂x but I can't see how either of them leads to the above. I almost understand it when I try to take the latter but apparently they take y and y/x^2 as constants and I don't understand why that is (assuming that is indeed what the solution is doing).