1/x-1/y=2(x-y)/xy=-2x-y=-2xy(3x-2xy-3y) / (5x+4xy-5y) (题目中的分母应为：(5x+4xy-5y),否则无法做）=[3(x-y)-2xy] /[5(x-y)+4xy]=(-6xy-2xy) / (-10xy-4xy)=-8xy / (-14xy)=4/7