| a |
| 4 |
| a |
| 4 |

解:(1)如图(1)所示:
观察规律:如图,PM1⊥M1M2,PM2⊥M2M3,PM3⊥M3M4,…,且PM1=M1M2=M2M3=M3M4=…=Mn-1Mn=1,那么PMn的长是| n |
| n |

|
| 1 |
| 2 |
|
|
| 1 |
| 3 |
|
|
| 1 |
| 4 |
|
|
| 1 |
| n+1 |
|
|
| 1 |
| n+1 |
|