题干
观察下列等式：
第1个等式：a1=
=
×1-
； 第2个等式：a2=
=
×
-
| 1 |
| 1 |
| × |
| 3 |
| 1 |
| 2 |
| 1 |
| 3 |
| 1 |
| 3 |
| × |
| 5 |
| 1 |
| 2 |
| 1 |
| 3 |
| 1 |
| 5 |
第3个等式：a3=
=
×
-
=
×
-
| 1 |
| 5 |
| × |
| 7 |
| 1 |
| 2 |
| 1 |
| 5 |
| 1 |
| 7 |
| 1 |
| 7 |
| × |
| 9 |
| 1 |
| 2 |
| 1 |
| 7 |
| 1 |
| 9 |
请解答下列问题：
