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90 (ninety) is the natural number preceded by 89 and followed by 91. In English speech, the numbers 90 and 19 are often confused, as sounding very similar. When carefully enunciated, they differ in which syllable is stressed: 19 /naɪnˈtiːn/ vs 90 /ˈnaɪnti/. However, in dates such as 1999, and when contrasting numbers in the teens and when counting, such as 17, 18, 19, the stress shifts to the first syllable: 19 /ˈnaɪntiːn/. == In mathematics == *Because 90 is the sum of its unitary divisors (excluding itself), it is a unitary perfect number, and because it is equal to the sum of a subset of its divisors, it is a semiperfect number. 90 is a pronic number. But it is also a nontotient. It is a Perrin number, preceded in the sequence by 39, 51, 68. *In normal space, the interior angles of a square measure 90 degrees each. Also, in a right triangle, the angle opposing the hypotenuse measures 90 degrees, with the other two angles adding up to 90 for a total of 180 degrees.〔(http://www.sparknotes.com/testprep/books/newsat/chapter20section4.rhtml )〕 Thus, an angle measuring 90 degrees is called a right angle.〔Friedman, Erich. (What's Special About This Number? )〕 *90 is divisible by the sum of its base 10 digits, thus it is a Harshad number. 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「90 (number)」の詳細全文を読む スポンサード リンク
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