由x分之1-y分之1=2得到,1/x-1/y=2,即 （y-x）/(xy)=2,即 x-y=-2xy (1).
2x-3xy-2y分之3x+7xy-3y= (2x-3xy-2y)/(3x+7xy-3y) (2)
将(1) 带入 (2)
其中(2x-3xy-2y)= 2x-2y -3xy=-2*2xy-3xy=-7xy
(3x+7xy-3y) = 3(x-y)+7xy=3*(-2xy)+7xy=xy
所以,(2x-3xy-2y)/(3x+7xy-3y) = -7xy/xy=-7.