已知；xy/x+y=2,求代数式3x+3y-5xy/-x+3x-y的值.谢谢

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xy=2(x+y)
所以原式=[3(x+y)-5xy]/[-(x+y)+3xy]
=[3(x+y)-5*2(x+y)]/[-(x+y)+3*2(x+y)]
=-7(x+y)/[5(x+y)]
=-7/5