????:1????????:??????sn

2????s

3??????

4??????

5?????

??

???????:7???? ?????????

??11???????17??????????19???????????,?????????,???????????????,??????????????????????????????????????????????????????????????????7????????,????

???,????????????,???“?????”??????????????????????????????????,????????“???????????”?????

?????,???????,??????,??,?????,??????,??????????

????????

????????????????????????????????????

(?-????????) ??:????