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+1-3=-1为极小值