已知xy/x+y=2,求3x-5x+3y/-x+3xy-y的值.

xy/(x+y)=2
x+y=1/2xy
(3x-5xy+3y)/(-x+3xy-y)
=[3(x+y)-5xy]/[-(x+y)+3xy]
=(3/2xy-5xy)/(-1/2xy+3xy)
=(3/2-5)/(3-1/2)
=-7/5
你是不是分子分母写倒了
若是xy 是分母
x+y是分子
(x+y)/xy=2
x+y=2xy
(-x+3xy-y)/(3x-5xy+3y)
=[-(x+y)+3xy]/[3(x+y)-5xy]
=(-2xy+3xy)/(6xy-5xy)
=xy/(xy)
=1
再问： xy是分子，x+y是分母。是3x-5y+3y，不是3x-5xy+3y
再答： 肯定是题目错了 应该是5xy xy/(x+y)=2 x+y=1/2xy (3x-5xy+3y)/(-x+3xy-y) =[3(x+y)-5xy]/[-(x+y)+3xy] =(3/2xy-5xy)/(-1/2xy+3xy) =(3/2-5)/(3-1/2) =-7/5 或者xy=2(x+y) 3x-5xy+3y/-x+3xy-y =[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
再问： 我问老师了，的确，谢谢你！