题干
设等差数列{an}，{bn}的前n项和分别为Sn，Tn若对任意自然数n都有
=
，则
+
的值为____．
| S |
| n |
| T |
| n |
| 2 |
| n |
| 3 |
| 4 |
| n |
| 3 |
| a |
| 9 |
| b |
| 5 |
| + |
| b |
| 7 |
| a |
| 3 |
| b |
| 8 |
| + |
| b |
| 4 |
