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

2??????????

3????????????

4??????

5???????sn

??,2?????????15???,????????????????????30??????????????(????????)????80???????????????1000?????3000???

???????,??????????????,????“???”,????2????3????????,?????????????????????,????????????????,???,??“????”“????”??????,??????????????

(??? ???? ??? ?? ???? ??????????????????)

?????????????????:??????????????????