2xy²-[3xy²-2(x²y-(1/2)xy²)]-2x²y
=（2-3）xy²+2x²y+(1/2)xy²-2x²y
=(1/2)xy²-xy²
=-(1/2)xy²
x=-2/3,y=-1/2
带入,得
-(1/2)×-2/3×（-1/2）²
=1/122(x²y-(1/2)xy²的2x²y+(1/2)xy²不应该是＋xy吗，乘以2奥2xy²-[3xy²-2(x²y-(1/2)xy²)]-2x²y=（2-3）xy²+2x²y-2×(1/2)xy²-2x²y=-xy²-xy²=-2xy²x=-2/3,y=-1/2带入，得-2×（-2/3）×（-1/2）²=1/3