a(2)=6,因为一个7 X 2矩形有6个平铺,使用右三角形和2 X 2平铺:
.___. .___. .___. .___. .___. .___.
| . | | . | | ._| | . | | . | |_. |
|___| |___| |_| | |___| |___| | |_|
| . | | ._| |___| | . | |_. | |___|
|___| |_| | | . | |___| | |_| | . |
| ._| |___| |___| |_. | |___| |___|
|_| | | . | | . | | |_| | . | | . |
|___| |___| |___| |___| |___| |___|