x=y=-1 ,xy=1; 2x²y-xy²+二分之一x²y-二分之三xy²+2x²y² =xy(2x-y+2xy)+(1/2)xy(x-3y)=(2x-y+2)+(1/2)(x-3y),【因为xy=1】=(-2+1+2)+(1/2)(1+3)=1+2=3