f'x=3x^2-8x+2y=0
f'y=2x-2y=0--> x=y,代入上式得：3x^2-6x=0,
即x=0 or 2,y=0 or 2
A=f"xx=6x-8
C=f"yy=-2
B=f"xy=2
当x=0,y=0,A=-2,B=2,C=-2,AC-B^2=-8*(-2)-4=12>0,A