| 1 |
| 1×4 |
| 1 |
| 4×7 |
| 1 |
| (3n-2)×(3n+1) |
则3Sn=
| 3 |
| 1×4 |
| 3 |
| 4×7 |
| 3 |
| (3n-2)×(3n+1) |
| 1 |
| 4 |
| 1 |
| 4 |
| 1 |
| 7 |
| 1 |
| (3n-2) |
| 1 |
| (3n+1) |
| 1 |
| (3n+1) |
| 3n |
| (3n+1) |
所以Sn=
| n |
| (3n+1) |
故答案为
| n |
| (3n+1) |
| 1 |
| 1×4 |
| 1 |
| 4×7 |
| 1 |
| (3n-2)×(3n+1) |
| 1 |
| 1×4 |
| 1 |
| 4×7 |
| 1 |
| (3n-2)×(3n+1) |
| 3 |
| 1×4 |
| 3 |
| 4×7 |
| 3 |
| (3n-2)×(3n+1) |
| 1 |
| 4 |
| 1 |
| 4 |
| 1 |
| 7 |
| 1 |
| (3n-2) |
| 1 |
| (3n+1) |
| 1 |
| (3n+1) |
| 3n |
| (3n+1) |
| n |
| (3n+1) |
| n |
| (3n+1) |