f'x=3x^2-3y=0--> y=x^2
f'y=3y^2-3x=0-->y^2=x
x=y^2=x^4--> x=0,1,-->y=0,1
f"xx=6x,f"yy=6y,f"xy=-3
f(0,0)=0不为极值
f(1,1)=-1为极小值