C (n,r) + C (n, r-1)
= n! /r! (n -r )! +n! / (r-1)! (n-r +1 )!
= n! (n- r +1 ) +n! *r / r! (n-r+1)! <-----怎樣不見了(n-r )! 和(r-1)!
= C (n+1 , r)
收錄日期: 2021-05-03 20:15:27
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20110512000051KK00109