7y(x-2y)平方-2(2y-x)立方
=7y(x-2y)平方+2(x-2y)立方
=(x-2y)²(7y+2x-4y)
=(x-2y)²(2x+3y)
=(10/7)²x49/8
=100/49x49/8
=100/8
=12.5
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