十八模民间小调视频:七年级数学问题!!!!!!!!!!!!!!!!!!!!!急!

来源：百度文库编辑：查人人中国名人网时间：2024/04/27 18:08:18

1.已知xy/x+y=2,求代数式3x-5xy+3y/-x+3xy-y的值?
(代数式是一起除的).

xy/(x+y)=2
得到xy=2(x+y)

(3x-5xy+3y)/(-x+3xy-y)
=[3x-5×2(x+y)+3y]/(-x+3×2(x+y)-y]
=(3-10)(x+y)/(-1+6)(x+y)
=-7/5

=1

xy=2(x+y)
3x-5xy+3y)/(-x+3xy-y)
=[3x-5×2(x+y)+3y]/(-x+3×2(x+y)-y]
=(3-10)(x+y)/(-1-6)(x+y)

把xy=2(x+y)代入3x-5xy+3y/(-x+3xy-y)
3x-5xy+3y/(-x+3xy-y)=-7(x+y)/5(x+y)=-7/5

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