A:一客失了信,百客不登门
B:日方中方睨,物方生方死
C:兴废由人事,山川空地形
D:古调虽自爱,今人多不弹
(2015•潍坊)如图,正△ABC的边长为2,以BC边上的高AB1为边作正△AB1C1,△ABC与△AB1C1公共部分的面积记为S1;再以正△AB1C1边B1C1上的高AB2为边作正△AB2C2,△AB1C1与△AB2C2公共部分的面积记为S2;…,以此类推,则Sn= ____ .(用含n的式子表示)
As the smallest child of his family, Alex is always longing for the time _______ he should be able to be independent.