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?
人气：413 ℃ 时间：2020-03-27 11:54:59
解答
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).
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