f(x,y)对x的偏导=4-2x=0,x=2f(x,y)对y的偏导=-4-2y=0,y=-2f(x,y)对x二阶导=-2f(x,y)对y二阶导=-2f(x,y)对x的偏导再对y的偏导=0因-2*-2-0^2=4>0，故(2,-2)是极值，f(x,y)对x二阶=-2<0，所以是极大值f(2,-2)=8
8
