z'x=b+dy

z'y=c+dx

驻点：（-c/d, -b/d)

A=0 B=d C=0 AC-B^2=-d^2<0 驻点非极值点

x=-e时 z=a-be+cy-dey=a-be+(c-de)y

函数最大值=max{a+be+ce+de^2,a+be-ce-de^2,a-be+ce-de^2,a-be-ce+de^2}

函数最小值=min{a+be+ce+de^2,a+be-ce-de^2,a-be+ce-de^2,a-be-ce+de^2}