?? ?????? ?????????????????????
????:1???????:?????????
2?????sn????
3??????????????
4????????
5??????
??12??????????20? ??,?,??????,?????????20????
?????????,???????????????,9????????5?????????????????,?????????12??16??????,?,?20?????????,??
????????????????,?????????????????,???,???,?????????????????????????????,??????????????????????????????????,?????(???????)
?????????:??????????????