xy/x+y=3
xy=3(x+y)
3x-5xy+3y/-x+3xy-y
=(3x+3y)-5*3(x+y)/[-x-y+3*3(x+y)]
=-12(x+y)/8(x+y)
=-3/2
望采纳，谢谢！
xy/x+y=3 取倒数 得 1/x+1/y=1/3
3x-5xy+3y/-x+3xy-y=(3/x+3/y-5)/[3-(1/x+1/y)]=(1-5)/(3-1/3)=(-4/(8/3)=-3/2(分子分母同除xy)
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