化简求职:2xy^2-[3xy^2-2(x^2y-(1/2)xy^2)]-2x^2y其中x=-2/3,y=-1/2

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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
=1/12
再问： 2(x²y-(1/2)xy²的2x²y+(1/2)xy²不应该是＋xy吗，乘以2
再答：奥 2xy²-[3xy²-2(x²y-(1/2)xy²)]-2x²y =（2-3）xy²+2x²y-2×(1/2)xy²-2x²y =-xy²-xy² =-2xy² x=-2/3,y=-1/2 带入，得-2×（-2/3）×（-1/2）²=1/3

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