The binomial theorem describes the algebraic expansion of powers of a binomial.
According to the theorem, it is possible to expand the power (x + y)^{n} into a sum involving terms of the form a^{x}b^{y}c, 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. When an exponent is zero, the corresponding power is usually omitted from the term.

In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n . For example,

The value of 0! is 1, according to the convention for an empty product. The factorial operation is encountered in many different areas of mathematics, notably in combinatorics, algebra, and mathematical analysis. Its most basic occurrence is the fact that there are n! ways to arrange n distinct objects into a sequence (i.e., permutations of the set of objects). The definition of the factorial function can also be extended to non-integer arguments, while retaining its most important properties; this involves more advanced mathematics, notably techniques from mathematical analysis.

The coefficients that appear in the binomial expansion are called binomial coefficients.
These are usually written

The coefficient of x^{n−k}y^{k} is given by the formula:

Note that, although this formula involves a fraction, the binomial coefficient