题干
设等差数列{an}的前n项和为Sn，若
=
，则
=____．
| a |
| 5 |
| a |
| 3 |
| 5 |
| 3 |
| s |
| 5 |
| s |
| 3 |
答案(点此获取答案解析）
| 5 |
| 2 |
