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280 (number) : ウィキペディア英語版 | 280 (number)
280 (two hundred () eighty) is the natural number after 279 and before 281. ==In mathematics== The denominator of the eighth harmonic number, 280 is an octagonal number. There are 280 plane trees with ten nodes. As a consequence of this, 18 people around a round table can shake hands with each other in non-crossing ways, in 280 different ways (this includes rotations). 280 is a base 10 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「280 (number)」の詳細全文を読む
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