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In mathematics, a square root of a number ''a'' is a number ''y'' such that , in other words, a number ''y'' whose ''square'' (the result of multiplying the number by itself, or ) is ''a''.〔Gel'fand, (p. 120 ) 〕 For example, 4 and −4 are square roots of 16 because . Every non-negative real number ''a'' has a unique non-negative square root, called the ''principal square root'', which is denoted by , where √ is called the ''radical sign'' or ''radix''. For example, the principal square root of 9 is 3, denoted = 3, because and 3 is non-negative. The term whose root is being considered is known as the ''radicand''. The radicand is the number or expression underneath the radical sign, in this example 9. Every positive number ''a'' has two square roots: , which is positive, and −, which is negative. Together, these two roots are denoted ± (see ± shorthand). Although the principal square root of a positive number is only one of its two square roots, the designation "''the'' square root" is often used to refer to the ''principal square root''. For positive ''a'', the principal square root can also be written in exponent notation, as ''a''1/2.〔, (Extract of page 78 )〕 Square roots of negative numbers can be discussed within the framework of complex numbers. More generally, square roots can be considered in any context in which a notion of "squaring" of some mathematical objects is defined (including algebras of matrices, endomorphism rings, etc.) ==History== The Yale Babylonian Collection YBC 7289 clay tablet was created between 1800 BC and 1600 BC, showing and 30 as 1;24,51,10 and 42;25,35 base 60 numbers on a square crossed by two diagonals.〔(【引用サイトリンク】title=Analysis of YBC 7289 )〕 The Rhind Mathematical Papyrus is a copy from 1650 BC of an earlier Berlin Papyrus and other texts – possibly the Kahun Papyrus – that shows how the Egyptians extracted square roots by an inverse proportion method.〔Anglin, W.S. (1994). ''Mathematics: A Concise History and Philosophy''. New York: Springer-Verlag.〕 In Ancient India, the knowledge of theoretical and applied aspects of square and square root was at least as old as the ''Sulba Sutras'', dated around 800–500 BC (possibly much earlier). A method for finding very good approximations to the square roots of 2 and 3 are given in the ''Baudhayana Sulba Sutra''.〔Joseph, ch.8.〕 Aryabhata in the ''Aryabhatiya'' (section 2.4), has given a method for finding the square root of numbers having many digits. It was known to the ancient Greeks that square roots of positive whole numbers that are not perfect squares are always irrational numbers: numbers not expressible as a ratio of two integers (that is to say they cannot be written exactly as ''m/n'', where ''m'' and ''n'' are integers). This is the theorem ''Euclid X, 9'' almost certainly due to Theaetetus dating back to circa 380 BC. The particular case is assumed to date back earlier to the Pythagoreans and is traditionally attributed to Hippasus. It is exactly the length of the diagonal of a square with side length 1. In the Chinese mathematical work ''Writings on Reckoning'', written between 202 BC and 186 BC during the early Han Dynasty, the square root is approximated by using an "excess and deficiency" method, which says to "...combine the excess and deficiency as the divisor; (taking) the deficiency numerator multiplied by the excess denominator and the excess numerator times the deficiency denominator, combine them as the dividend."〔Dauben (2007), p. 210.〕 Mahāvīra, a 9th-century Indian mathematician, was the first to state that square roots of negative numbers do not exist. A symbol for square roots, written as an elaborate R, was invented by Regiomontanus (1436–1476). An R was also used for Radix to indicate square roots in Gerolamo Cardano's Ars Magna.〔(【引用サイトリンク】title=The Development of Algebra - 2 )〕 According to historian of mathematics D.E. Smith, Aryabhata's method for finding the square root was first introduced in Europe by Cataneo in 1546.〔Smith, p. 148.〕 The symbol '√' for the square root was first used in print in 1525 in Christoph Rudolff's ''Coss'' (which was also the first to use the then-new signs, '+' and '−'). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「square root」の詳細全文を読む スポンサード リンク
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