| 1 |
| 1+2+3+…+n |
| 2 |
| n(n+1) |
∴Sn=a1+a2+a3+…+an
=2（
| 1 |
| 1×2 |
| 1 |
| 2×3 |
| 1 |
| 3×4 |
| 1 |
| n×(n+1) |
=2×(1−
| 1 |
| 2 |
| 1 |
| 2 |
| 1 |
| 3 |
| 1 |
| 3 |
| 1 |
| 4 |
| 1 |
| n |
| 1 |
| n+1 |
=2（1-
| 1 |
| n+1 |
| 2n |
| n+1 |
故答案：
| 2n |
| n+1 |
| 1 |
| 1+2 |
| 1 |
| 1+2+3 |
| 1 |
| 1+2+3+…+n |
| 1 |
| 1+2+3+…+n |
| 2 |
| n(n+1) |
| 1 |
| 1×2 |
| 1 |
| 2×3 |
| 1 |
| 3×4 |
| 1 |
| n×(n+1) |
| 1 |
| 2 |
| 1 |
| 2 |
| 1 |
| 3 |
| 1 |
| 3 |
| 1 |
| 4 |
| 1 |
| n |
| 1 |
| n+1 |
| 1 |
| n+1 |
| 2n |
| n+1 |
| 2n |
| n+1 |