化简求值:3x2y-[2x2y-3(2xy-x2y)-xy],其中x=-1,y=-2
3x²y-[2x²y-3﹙2xy-x²y﹚-xy]
=3x²y-[2x²y-6xy+3x²y-xy]
=3x²y-[5x²y-7xy]
=3x²y-5x²y+7xy
=-2x²y+7xy
=xy﹙-2x+7﹚
把x=-1,y=-2代入上式得:
=﹙-1﹚×﹙-2﹚[﹙-2﹚×﹙-1﹚+7]
=2×[2+7]
=2×9
=18
3x²y-[2x²y-3(2xy-x²y)-xy]
=3x²y-2x²y+3(2xy-x²y)+xy
=x²y+(6xy-3x²y)+xy
=-2x²y+7ab
【x=-1,y=2】
=-2(-1)²(-2)+7(-1)(-2)
=4+14
=18
【x=-12,y=2】
=-2(-12)²*2+7(-12)*2
=-576-168
=-744
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