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

2?????sn??

3????????

4????????

5???????

????????11??23?????????? (??????? ?????)????23??,??????????,24??????25?????????,????????????????,???????????,????,???????????,25?????25????????,???

????????????????,?????????????,25?????????????27??????????,??3?4?,?????????????6?????25??27?????,??????26?????????????27??3?,27???-5??

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