Question

I am working on second derivative tests having but having trouble with finding the critical points. The problem is find the critical points of f(x,y)=sinx+siny+cos(x+y) for x between 0 and pie/4 and y between 0 and pie/4. Classify each as a local min, max, or saddle point. f(x,y)=sinx+siny+cos(x+y) I found the partials to be: derivative of f with respect to x = fx=cosx-sin(x+y) derivative of f with respect to y = fy=cosy-sin(x+y) I am having trouble solving the partials for 0.