????:1???????????:????????

2?????????

3??????????

4????????

5???sn??

????2???????17?????? (?? ??)??17??????,??,???????????????10??

???????????????,17?,??,18???????????????????,19???,??20??????????????21???

????????,18?,?,??????????????,??????????????????:??8??????????10??

19????,?,???????????????:??????????????8????????????12??

??????,???????,????????,????????????(????) ???????:??