x分之一减y分之一等于三
去分母得 y-x=3xy
3x-3y分之x-3x-3y分之y+2xy
=(3x-3y)分之[x-(y+2xy)]
=3(x-y)分之（x-y-2xy)
=-3×3xy分之(-3xy-2xy)
=9分之5