多项分布概率公式的理解

http://crescentluna.github.io/2012/09/23/%E5%A4%9A%E9%A1%B9%E5%88%86%E5%B8%83%E6%A6%82%E7%8E%87%E5%85%AC%E5%BC%8F%E7%9A%84%E7%90%86%E8%A7%A3/#more-9

http://baike.baidu.com/view/1906564.htm