[b]uFactorial notation[/u]/b

The notation (n!) (pronounced (n) factorial) where (n) is an integer means (n) multiplied by ((n-1)) multiplied by ((n-2)) etc. all the way down to 1.

(n!=n×(n-1)×(n-2)×...×2×1)

[b]uCombinations[/u]/b

The number of ways of choosing (r) items from (n) items is written as (^nC_r) or (\dbinom{n}{r}).

(^nC_r = \dbinom{n}{r} = \dfrac{n!}{r!(n-r)!})

(n!=n×(n-1)×(n-2)×...×2×1)

(^nC_r = \dbinom{n}{r} = \dfrac{n!}{r!(n-r)!})