So we first find where fx(x,y) = 0 and fy(x,y) = 0, where fx and fy are the partial derivatives of z = f(x,y). Once we find those critical points, we use D = (fxx)(fyy) - (fxy)^2. If D > 0 and fxx > 0, we have a local min at that point. If D > 0 and fxx < 0, we have a local max at that point. If D < 0, we have a saddle point. If D = 0, no information can be found using the second derivative test. My question is: 1. How do we deal with the D = 0 situation? How would we find if that point's a max or min? 2. What if fxx = 0?