题干
已知数列1，
,
,⋯
,⋯，则其前n项的和等于____
| 1 |
| 1 |
| + |
| 2 |
| 1 |
| 1 |
| + |
| 2 |
| + |
| 3 |
| 1 |
| 1 |
| + |
| 2 |
| + |
| 3 |
| + |
| ⋯ |
| + |
| n |
答案(点此获取答案解析）
| 2 |
| n |
| n |
| + |
| 1 |
| 1 |
| 1 |
| + |
| 2 |
| 1 |
| 1 |
| + |
| 2 |
| + |
| 3 |
| 1 |
| 1 |
| + |
| 2 |
| + |
| 3 |
| + |
| ⋯ |
| + |
| n |
| 2 |
| n |
| n |
| + |
| 1 |
