?? ????15?????????? ???50????
????:1???????:?????????
2??????
3??????
4???????????
5??????????
?6???21? ???(ABC)?????????????,??20???????,????,?????19????????????,??????15?????????
????????????:??????????(ABC)???
?????????,???????,?????????????????????????????????????????????????????????,???????????
???,??????,????,???????????????20???30???????,??????????????50?????
??,???????
?,????????,????
???????:?????