1/X-1/Y=3
(2X+XY-2Y) / (X-2XY-Y)
分子分母同除以XY：
= (2/Y+1-2/X) / (1/Y-2-1/X)
= {1-2(1/X-1/Y)} / {-2-(1/X-1/Y)}
= {1-2*3} / {-2-3}
= (-5) / (-5)
=1