1/x-1/y=3
y/xy-x/xy=3
y-x=3xy
x-y=-3xy
3x-5xy-3y/x-2xy-y
=[3(x-y)-5xy]/[(x-y)-2xy]
=(-9xy-5xy)/(-3xy-2xy)
=14/5麻烦问一下 为什么y-x=3xy呢？y/xy-x/xy=3去分母得y-x=3xy