z'(x)=3x^2-8x+2y z'(y)=2x-2y 解得驻点为x1=1,y1=1；x2=2,y2=2,.然后分别求,这里只求一个,z''(x)=6x-8,z''(y)=-2,z'(xy)=2,A=-2 B=2 C=-2 A
