【备注：2x-3y-2y/x-2xy-y 应为(2x-3xy-2y)/(x-2xy-y)
1/x-1/y=3
(2x-3xy-2y)/(x-2xy-y)
分子分母同除以xy：
= (2/y-3-2/x)/(1/y-2-1/x)
= {-3-2(1/x-1/y)} / {-2-(1/x-1/y)}
= {-3-2*3}/{-2-3}
= -9/(-5)
= 9/5
= 1.8