题干
如图，已知正四棱锥P﹣ABCD中，PA=AB=2，点M，N分别在PA，BD上，且
=
=
．
| P |
| M |
| P |
| A |
| B |
| N |
| B |
| D |
| 1 |
| 3 |
| P |
| M |
| P |
| A |
| B |
| N |
| B |
| D |
| 1 |
| 3 |
