1. The problem statement, all variables and given/known data Locate and classify the critical points of f(x,y) = (x-y)(xy-1). 2. Relevant equations 3. The attempt at a solution I found the partial derivatives with respect to x and y and I got: ∂f/∂x = -y2+2xy-1 ∂f/∂y = x2-2xy+1 After setting them both equal to zero I can't find a way to solve for x and y.