Probability & Combinatorics
Basic

Combinations

Number of unordered selections of r items from n.

Formula

C(n,r)=(nr)=n!r!(n−r)!C(n,r) = \binom{n}{r} = \frac{n!}{r!(n-r)!}

Variables

nTotal items
rChosen items

Example

C(5,2) = 10

Did You Know?

The binomial coefficient counts subsets and appears in Pascal triangle.

countingselection