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

????:1?????:?????????
2??????
3????????????
4??????vx
5???????
??12??18?(?? ?? ????????????)12??17??,?,?21?,??????????????????,????(??????????????),??,?4???

??,?????????????????????????????????????????????????????????????????????????????????????????????????????12????????????,???????????24??(?????)???
??????????,???????????,???,?????,??,?????????????????
??,?????????????????????????????240???????????????,?????????????;?????????????????????,????????????????(?????)
???????????:????