f'x=y(2-x-y)-xy=y(2-2x-y)=0,得y=0或y=2-2xf'y=x(2-x-y)-xy=x(2-x-2y)=0,得x=0或x=2-2y解得有以下几组(0,0), (2,0), (0, 2), (2/3,2/3)A=f"xx=-2yB=f"xy=2-2x-2yC=f"yy=-2x在(0,0), AC-B^2=0-2^2=-4