HMMT 二月 2000 · POW 赛 · 第 7 题
HMMT February 2000 — POW Round — Problem 7
题目详情
英文原题
- Find the number of om binations of length k that use elemen ts from a giv en set of ndistin t elemen ts, allo wing rep etition and with no missing elemen ts. (Ob viously , k m ustbe greater than n )
解析
英文解析
- W e an use the same metho d as the ab o v e argumen t, ex ept that we kno w ev ery elemen to urs at least on e. So, this is the same as ho osing k n times from the n distin tk n n +1 k 1 k 1
ob je ts, so the answ er is , or or .
n 1
n 1 k n
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