由1/x -1/y=3两边同乘以xy得
y-x=3xy
即x-y=-3xy
所以(3x+3xy-3y)/(x-2xy-y)
＝3(x-y+xy)/(x-y-2xy)
=3(-3xy+xy)/(-3xy-2xy)
=3(-2xy)/(-5xy)
=6/5