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In elementary algebra, the binomial theorem describes the algebraic expansion of powers of a binomial. According to the theorem, it is possible to expand the polynomial into a sum involving terms of the form axy, where the exponents b and c are nonnegative integers with b + c = n, and the coefficient a of each term is a specific positive integer depending on n and b. For example, for n = 4,
The coefficient a in the term of axy is known as the binomial coefficient {\displaystyle {\tbinom {n}{b}}} or {\displaystyle {\tbinom {n}{c}}} . These coefficients for varying n and b can be arranged to form Pascal's triangle. These numbers also occur in combinatorics, where {\displaystyle {\tbinom {n}{b}}} gives the number of different combinations of b elements that can be chosen from an n-element set. Therefore {\displaystyle {\tbinom {n}{b}}} is often pronounced as "n choose b".