1/x-1/y=5
(y-x)/xy=5
y-x=5xy
(2x+5xy-2y)/(x-2xy-y)+1
=[5xy+2(x-y)]/[(x-y)-2xy]+1
=-5xy/(-7xy)+1
=5/7+1
=12/7-5XY/-7XY怎么来的？第三步 y-x=5xy x-y=-5xy5xy+2(x-y)=5xy-2×5xy=-5xy(x-y)-2xy=-5xy-2xy=-7xy