如何计算Combinations Permutations

learn.whatIsHeading

Combinations count ways to choose items where order does not matter. Permutations count ways where order does matter. Both use factorials and appear in probability, statistics, and combinatorics.

分步指南

1. 1Permutations (order matters): P(n,r) = n! / (n−r)!
2. 2Combinations (order irrelevant): C(n,r) = n! / (r!(n−r)!)
3. 3Rule of thumb: if you can swap two items and get a different result, use permutations

例题解析