Each person at a party shook hands exactly once with each of the other people at the party.There was a total of 21 hands
问题描述：
Each person at a party shook hands exactly once with each of the other people at the party.There was a total of 21 handshakes exchanged at the party.What is the number of people at the party?
答
排列组合问题吧.7*6/2＝21 故有7个人.就是那个C7(上标)6(下标).