Each person at a party shook hands exactly once with each other people at the party.There was a total of 21 handshakes exchanged at the party,问party上有几个人?请解释详细点,数学不好,谢谢
这个题你会用组合的方式解吗?
是n!/(n-2)!*2!=21 !是指从这个数一直乘到一,比如2!=2*1,5!=5*4*3*2*1可解n=7