(2x-3xy-2y)/(x-2xy-y)
=[(2/y)-3-(2/x)]/[(1/y)-2-(1/x) (分子分母同时乘以1/(xy))
=[2(1/x-1/y)-3]/[(1/x-1/y)-2]
∵1/x-1/y=3
∴原式=(6-3)/(3-2)=3