先化简，后求值：5x2y+{xy−[5x2y−(7xy2+1/2xy)]−(4x2y+xy)}−7xy2，其中x＝−1/4，y=-16．

原式=5x²y+{xy-[5x²y-7xy²-

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xy]-4x²y-xy}-7xy²
=5x²y+{xy-5x²y+7xy²+

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2

xy-4x²y-xy}-7xy²
=5x²y+xy-5x²y+7xy²+

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xy-4x²y-xy-7xy²
=

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xy，
当x=-

1

4

，y=-16，原式=

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×（-

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）×（-16）=2．