suppose an unknown function f(x,y) has critical points (a,b) and (c,d). We do not know the points, but we do know the discriminant and fsubxx at each point: D(a,b)=-13, fxx(a,b)=-4, D(c,d)=21, fxx(c,d)=-6. Use this information to classify both points as a local min, local max, saddlepoint, or state that the second derivative test is inconclusive.