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Combinations are studied in combinatorics: let S be a set; the combinations of this set are its subsets. A k-combination is a subset of S with k elements. The order of listing the elements is not important in combinations: two lists with the same elements in different orders are considered to be the same combination. The number of k-combinations of set with n elements is the binomial coefficient "n choose k", written as nCk, nCk or as
<math>{n \choose k},</math>
or occasionally as C(n, k).

One method of deriving a formula for nCk proceeds as follows:

  1. Count the number of ways in which one can make an ordered list of k different elements from the set of n. This is equivalent to calculating the number of k-permutations.
  2. Recognizing that we have listed every subset many times, we correct the calculation by dividing by the number of different lists containing the same k elements:

<math> {n \choose k} = \frac{P(n,k)}{P(k,k)} </math>


<math> P(n,k) = \frac{n!}{(n-k)!} </math>
(see factorial), we find

<math> {n \choose k} = \frac{n!}{k! \cdot (n-k)!} </math>

It is useful to note that C(n, k) can also be found using Pascal's triangle, as explained in the binomial coefficient article.

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