若1/X-1/Y=3,求5X+2XY-5Y/X-4XY-Y
问题描述：
若1/X-1/Y=3,求5X+2XY-5Y/X-4XY-Y
答
因为1/x-1/y=3
两边乘以xy得y-x=3xy, x-y=-3xy
所以原式=[5(x-y)+2xy]/(x-y-4xy)
=(-5*3xy+2xy)/(-3xy-4xy)
=(-13xy)/(-7xy)
=13/7