?? ?????????8?? ?????????2.2???
????:1???????:??????
2????????sn
3??????
4???????????
5???????
????????????9????19????? (? ??????????????????????)??19??,??????????????????8??????,??????2.2???(???????,?)?
??????,???20???«??????????»«???????????????????????????»?????????????,???2.2???
??,??????,??,???
?????????47????????,?????????????????????????????????????????????8?????????
??,?????8?????,?????????????????????????????????(??????????) ????:????