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

2?????

3?????????

4????????vx

5???????

?????????????6?21?????????????  ???21??????,??????????????????????,?????????,????“????????????????”?????????????????????????

?????????????,??????,????????????????????3.6???????,????????????

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

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

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