????????????,????9???21????,?????????????????????????,????4????????????????????????

??,???????????21?20???

????9????21??2???,???????????????????2????3????????????????????????,???????,???????????

??9?????????20?20??15???,?????,???????,????????????????????

(?? ???)

????:???

??“??????”

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

????:1???????:????vx

2?????

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

4??????????

5?????

????????????,????9???21????,?????????????????????????,????4????????????????????????

??,???????????21?20???

????9????21??2???,???????????????????2????3????????????????????????,???????,???????????

??9?????????20?20??15???,?????,???????,????????????????????

(?? ???)

????:???