已知1/x-1/y=3
则（y－x）/xy＝3
即x－y＝-3xy
代入
(2x-14xy-2y)/(x-2xy-y)
＝2[(x-y)-7xy]/[(x-y)-2xy]
＝2[-3xy-7xy]/[-3xy-2xy]
＝-2×10xy/[-5xy]
＝4