题干
若等差数列 {an},{bn} 的前n项和分别为 Sn,Tn ，且
=
，则
= ____．
| S |
| n |
| T |
| n |
| 2 |
| n |
| + |
| 2 |
| 5 |
| n |
| + |
| 3 |
| a |
| 2 |
| + |
| a |
| b |
| 7 |
| + |
| b |
