已知xy/x+y=2,求3x-5x+3y/-x+3xy-y的值.

xy/(x+y)=2x+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=2x+y=2xy(-x+3xy-y)/(3x-5xy+3y)=[-...xy是分子，x+y是分母。是3x-5y+3y，不是3x-5xy+3y肯定是题目错了 应该是5xyxy/(x+y)=2x+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我问老师了，的确，谢谢你！