（Ⅰ）证明：记rn为圆On的半径，则r1＝
| l |
| 2 |
| ||
| 6 |
| rn−1−rn |
| rn−1+rn |
| 1 |
| 2 |
所以rn＝
| 1 |
| 3 |
于是a1＝π
| r | 12 |
| πl2 |
| 12 |
| an |
| an−1 |
| rn |
| rn−1 |
| 1 |
| 9 |
故{an}成等比数列．
（Ⅱ）因为an＝(
| 1 |
| 9 |
所以
| lim |
| n→∞ |
| a1 | ||
1−
|
| 3πl2 |
| 32 |
| lim |
| n→∞ |
（Ⅰ）证明：记rn为圆On的半径，| l |
| 2 |
| ||
| 6 |
| rn−1−rn |
| rn−1+rn |
| 1 |
| 2 |
| 1 |
| 3 |
| r | 12 |
| πl2 |
| 12 |
| an |
| an−1 |
| rn |
| rn−1 |
| 1 |
| 9 |
| 1 |
| 9 |
| lim |
| n→∞ |
| a1 | ||
1−
|
| 3πl2 |
| 32 |