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

2?????sn

3?????????

4????????????

5???????

??????9????????????20?? (???????????? ??????????? ???????????)?(??????)??“?”????????????????????20??????????,?????????????????????,?????

??,??;??,??????????,??????????,???????????????

9??????????????????20??,“????????”????????????“?”? ?????? ????? ????

???????????????“??????????????????”??????????,?“??+????+?????”??,?100?????????????/??????,?“????????????”??????????“???????”???“?????”????????????(???????????)???????“?”????“??”??????

????,???????«??»?????“??”?«????»?????«?»,?????1000??????,????????????????????????????????????????????????,??

???,«??????????????»???????????:??????????????;??,?????????,?;???????????«?»??

“?????”?????????? ?????? ?? ??

???????6??,??????????“?????”???????285???????“??”???????????????????320??“????????????????”??17??????????????3?????????,???????

??????,???????????6????????????????????,“????,??????????????????,?????????????????????????????????????,????????????,???,????,??”

“???????????”????12?????????????????20??(???)

???????:???