原式=-x³y²+2x4次方y²+4x³y²
=3x³y²+2x4次方y²
把x=2,y=-1代入得
原式=3×2³（-1）²+2×2的4次方×（-1）²
=24+32
=56
先化简再求值-1/2x²×(2xy²-4x²y²)-4x²y×（-xy
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