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Counting: permutations and combinations

Factorials, ordered and unordered selections.

n! counts the orderings of n things. Choosing k of them in order is n!/(n − k)!; ignoring the order divides by k! again, giving the binomial coefficient “n choose k”. These numbers are Pascal's triangle and the coefficients of (a + b)ⁿ.

Worked example: 5 choose 2

Arithmetic · Arithmetic

Evaluate 10

10

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  1. 10 = 10

    Evaluate.

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10

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