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In geometry, a decagon is a 10-sided polygon or 10-gon.〔.〕 ==Regular decagon== A ''regular decagon'' has all sides of equal length and each internal angle will always be equal to 144°.〔 Its Schläfli symbol is 〔.〕 and can also be constructed as a truncated pentagon, t, a quasiregular decagon alternating two types of edges. The area of a regular decagon is: (with ''t'' = edge length)〔. Note that this source uses ''a'' as the edge length and gives the argument of the cotangent as an angle in degrees rather than in radians.〕 : and it can be written algebraically as : 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「decagon」の詳細全文を読む スポンサード リンク
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