Permutations Formula

也称为: nPr Formula, Arrangement Formula

P(n, r) = \frac{n!}{(n-r)!}

P(n,r) = n! / (n-r)!

它计算什么

Counts the number of ways to arrange r items from a set of n, where ORDER MATTERS.

象征	姓名	单元	描述
n	Total Items	—	Total number of items to choose from
r	Items Chosen	—	Number of items being arranged

1P(5,3) = 5! / (5-3)! = 5! / 2!

2= 120 / 2 = 60

结果: 60

Permutations = order matters. Combinations = order doesn't matter.