∴得
| a1(q4−1) |
| q−1 |
| a1(q8−1) |
| q−1 |
由 ①和②式
整理得
| q8−1 |
| q4−1 |
解得q4=16
所以q=2或q=-2
将q=2代入 ①式得a1＝
| 1 |
| 15 |
∴a＝
| 2n−1 |
| 15 |
将q=-2代入 ①式得a1＝−
| 1 |
| 5 |
∴an＝
| (−1)n×2n−1 |
| 5 |
综上所述an＝
| 2n−1 |
| 15 |
| (−1)n×2n−1 |
| 5 |
| a1(q4−1) |
| q−1 |
| a1(q8−1) |
| q−1 |
| q8−1 |
| q4−1 |
| 1 |
| 15 |
| 2n−1 |
| 15 |
| 1 |
| 5 |
| (−1)n×2n−1 |
| 5 |
| 2n−1 |
| 15 |
| (−1)n×2n−1 |
| 5 |