What is a real number?

In mathematics, a real number is a value that represents a quantity along a line. The adjective real in this context was introduced in the 17th century by Descartes, who distinguished between real and imaginary roots of polynomials. The real numbers include all the rational numbers, such as the integer 5 and the fraction 4/3, and all the irrational numbers, such as 2 (1.41421356, the square root of 2, an irrational algebraic number). Included within the irrationals are the transcendental numbers, such as (3.14159265). Real numbers can be thought of as points on an infinitely long line called the number line or real line, where the points corresponding to integers are equally spaced. Any real number can be determined by a possibly infinite decimal representation, such as that of 8.632, where each consecutive digit is measured in units one-tenth the size of the previous one. The real line can be thought of as a part of the complex plane, and complex numbers include real numbers.

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