1/x-1/y=3
方程两边同乘以xy得：
y-x=3xy
即x-y=-3xy,把它带入所求的式子得：
（5x+2xy-5y）/(x-4xy-y)
=[5(x-y)+2xy]/[(x-y)-4xy]
=[5(-3xy)+2xy]/(-3xy-4xy)
=-13xy/(-7xy)
=13/7