Combinations (Leaving Cert Mathematics): Flashcards

Ways to select items without considering order

Order doesn't matter in combinations; does in permutations

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

Total number of items

Number of items to select

Product of all positive integers up to that number

$5 \times 4 \times 3 \times 2 \times 1 = 120$

$120$ ways

$3003$ ways

Selecting groups, teams, subsets where order is irrelevant

Factorials