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

2????????????

3?????????

4?????????

5?????

??????6??15?? (?????????????? ???)???14???????,?????6????????????????12?,?1000??????,??????,?20??????

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

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

??????,????,?“?????”,???,??????????“??????”?????(??????) ??????????????????????:?????