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The number of selections of n objects taken r at a time (that is, the number of ways of selecting r objects out of n) is denoted bynCr and is equal to

(See binomial coefficient, where the alternative notation is defined.) For example, from four objects A, B, C and D, there are six ways of selecting two: AB, AC, AD, BC, BD, CD. The property thatn+1Cr=nCr−1+nCr can be seen displayed in Pascal's triangle.

Subjects: Mathematics.

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