f'x=3x^2-3y
f'y=3y^2-3x
f'x=0,f'y=0
即x^2-y=0
y^2-x=0
消去y x^4-x=0
即x(x-1)(x^2+x+1)=0
x=0或1
y=0或1
x=y=0时f(x,y)=0
x=y=1时f(x,y)=-1