The equation for z = f(x,y) is:[br][br][math]z=0.15x^2-0.25y^2+1.[/math][br][br]At the critical point, the curvature in the x direction is increasing (concave up), and the curvature in the y direction is decreasing (concave down).[br][br]In addition, [math]\frac{\partial^2z}{\partial x\partial y}=0[/math], and so the critical point is a saddle point.