题干
观察下列等式：
①
=1﹣
，②
=
﹣
，③
=
﹣
．
| 1 |
| 1 |
| × |
| 2 |
| 1 |
| 2 |
| 1 |
| 2 |
| × |
| 3 |
| 1 |
| 2 |
| 1 |
| 3 |
| 1 |
| 3 |
| × |
| 4 |
| 1 |
| 3 |
| 1 |
| 4 |
将以上三个等式两边分别相加，得
| 1 |
| 1 |
| × |
| 2 |
| 1 |
| 2 |
| × |
| 3 |
| 1 |
| 3 |
| × |
| 4 |
| 1 |
| 2 |
| 1 |
| 2 |
| 1 |
| 3 |
| 1 |
| 3 |
| 1 |
| 4 |
| 1 |
| 4 |
| 3 |
| 4 |
| 1 |
| 1 |
| × |
| 2 |
| 1 |
| 2 |
| 1 |
| 2 |
| × |
| 3 |
| 1 |
| 2 |
| 1 |
| 3 |
| 1 |
| 3 |
| × |
| 4 |
| 1 |
| 3 |
| 1 |
| 4 |
| 1 |
| 1 |
| × |
| 2 |
| 1 |
| 2 |
| × |
| 3 |
| 1 |
| 3 |
| × |
| 4 |
| 1 |
| 2 |
| 1 |
| 2 |
| 1 |
| 3 |
| 1 |
| 3 |
| 1 |
| 4 |
| 1 |
| 4 |
| 3 |
| 4 |
