I am looking at the Max/Min problems section of the Princeton Review book and there is mention of the second derivative test involving calculation of the Hessian Matrix of a function f(x,y). Apparently, the following conclusions can be made:

if Hessian > 0 and d/dxdx (f) @ point P < 0 then f attains a max at P

if Hessian > 0 and d/dxdx (f) @ point P > 0 then f attains a min at P

if Hessian < 0, then f has a saddlepoint at P

if Hessian = 0, then no conclusion can be drawn

My only question is, when would I replace d/dxdx with d/dydy when drawing my conclusions? There is nothing about the problem that explains looking only at the second derivative with respect to x of f.

Just in case if you were wondering, I am using the Princeton Review 'Cracking the GRE Math Subject Test 3rd ed."

Thank you in advance.