1. The problem statement, all variables and given/known data So fx is how much f changes when you change x. Thus fxx is the rate of change of fx, or geometrically how fast the functions slope is changing. The same can be said for fy and fyy. But what about fxy and fyx? Could someone please explain to me what they mean? I want to understand what it means to understand the second derivative test: http://dl.dropbox.com/u/64325990/MATH%20253/Capture.PNG [Broken] I am not sure what the meaning of the term [fxy(a,b)]^2 is. I can't seem to visualize what's going on when you take the second derivative test and the textbook doesn't have a proof.