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In geometry, a hendecagon (also undecagon〔.〕〔 or endecagon〔.〕) is an 11-sided polygon or 11-gon. (The name ''hendecagon'', from Greek ''hendeka'' "eleven" and ''gon–'' "corner", is often preferred to the hybrid ''undecagon'', whose first part is formed from Latin ''undecim'' "eleven".〔(Hendecagon – from Wolfram MathWorld )〕) ==Regular hendecagon== A ''regular hendecagon'' is represented by Schläfli symbol . A regular hendecagon has internal angles of 147.27 degrees.〔.〕 The area of a regular hendecagon with side length ''a'' is given by〔.〕 : As 11 is not a Fermat prime, the regular hendecagon is not constructible with compass and straightedge.〔As Gauss proved, a polygon with a prime number ''p'' of sides can be constructed if and only if ''p'' − 1 is a power of two, not true for 11. See .〕 Because 11 is not a Pierpont prime, construction of a regular hendecagon is still impossible even with the usage of an angle trisector. Close approximations to the regular hendecagon can be constructed, however. For instance, the ancient Greek mathematicians approximated the side length of a hendecagon inscribed in a unit circle as being 14/25 units long.〔.〕 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「hendecagon」の詳細全文を読む スポンサード リンク
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