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

2???sn??

3?????????

4????????vx

5??????????

????2?21??? (????? ???)2?21?,?????,????????;?????????????????????????

???,??????????????,1977?????11?????????????,????????????,????????

?????,1973?????????5?,?????????,???????????????????????????????????,??????????????????????

?,1971??2???????????????????,??????????????????????,?????????????????????????(????????????????) ????:???