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tetracontagon : ウィキペディア英語版
tetracontagon

In geometry, a tetracontagon or tessaracontagon is a forty-sided polygon or 40-gon.〔.〕〔(''The New Elements of Mathematics: Algebra and Geometry ) by Charles Sanders Peirce (1976), p.298〕 The sum of any tetracontagon's interior angles is 6840 degrees.
==Regular tetracontagon==
A ''regular tetracontagon'' is represented by Schläfli symbol and can also be constructed as a truncated icosagon, t, which alternates two types of edges. Furthermore, it can also be constructed as a twice-truncated decagon, tt, or a thrice-truncated pentagon, ttt.
One interior angle in a regular tetracontagon is 171°, meaning that one exterior angle would be 9°.
The area of a regular tetracontagon is (with )
:A = 10t^2 \cot \frac
and its inradius is
:r = \fract \cot \frac
The factor \cot \frac is a root of the octic equation x^ - 8x^ - 60x^ - 8x^ + 134x^ + 8x^ - 60x^ + 8x + 1.
The circumradius of a regular tetracontagon is
:R = \fract \csc \frac
As 40 = 23 × 5, a regular tetracontagon is constructible using a compass and straightedge.〔(Constructible Polygon )〕 As a truncated icosagon, it can be constructed by an edge-bisection of a regular icosagon. This means that the values of \sin \frac and \cos \frac may be expressed in radicals as follows:
:\sin \frac = \frac(\sqrt-1)\sqrt(2+\sqrt)(5+\sqrt)}-\frac\sqrt)(\sqrt-1)
:\cos \frac = \frac(\sqrt-1)\sqrt-1)+\frac(1+\sqrt)\sqrt(2-\sqrt)(5+\sqrt)}

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