已知xy/x+y=2,求代数式3x-xy+3y/-x+3xy-y的值
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解：原式=(3x+3y-xy)/[3xy-(x+y)]
=[3(x+y)-xy]/[3xy-(x+y)]
=[3-xy/(x+y)]/[3xy/(x+y)-1]
=(3-2)/(3×2-1)
=1/5
=[3(x+y)-xy]/[3xy-(x+y)]
=[3-xy/(x+y)]/[3xy/(x+y)-1]
=(3-2)/(3×2-1)
=1/5
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