已知xy/x+y=3,求2x-3xy+2y/-x+3xy-y的值

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已知xy/x+y=3,求2x-3xy+2y/-x+3xy-y的值
已知xy/x+y=3,求2x-3xy+2y/-x+3xy-y的值

已知xy/x+y=3,求2x-3xy+2y/-x+3xy-y的值
答案：-7/8
1,首先用已知条件知道xy=3(x+y)
2,将3(x+y) 带入式子中,即2x-3(3x+3y)+2y/-x+3(3x+3y)-y
3,将式子分解,相约.则等于-7/8

如果是恒等的话~
赋值算一下就OK了~
比如：
x=1;y=2

xy/x+y=3
xy=3(x+y)
2x-3xy+2y/-x+3xy-y
=2x+2y-3*3(x+y)/-x-y+3*3(x+y)
=2(x+y)-9(x+y)/(-(x+y)+9(x+y)
=-7(x+y)/8(x+y)
=-7/8

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