题干
附加题：
+
+
=1−
+
−
+
−
=1−
=
．
观察下列等式：
=1−
，
=
−
，
=
−
，
| 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 |
