已知1/x - 1/y = 2,求3x+7xy-3y / 2x-3xy-2y的值
1/x-1/y=2
(y-x)/xy=2 得：x-y=-2xy
(3x+7xy-3y)/(2x-3xy-2y)
=[3(x-y)+7xy]/[2(x-y)-3xy]
=(-6xy+7xy)/(-4xy-3xy)
=xy/(-7xy)
=-1/7
1/x-1/y=2到(y-x)/xy=2 是怎么变化的..