Decagon

A decagon

A decagon is a shape with 10 sides and 10 corners.

Regular decagon

All sides of a regular decagon are the same length. Each corner is 144°. All corners added together equal 1440°.

Area

The amount of space a regular decagon takes up is

[math]\displaystyle{ \text{Area} = \frac{5}{2} a^2\sqrt{5+2\sqrt{5}}. }[/math]

a is the length of one of its sides.

An alternative formula is [math]\displaystyle{ A = 2.5dt }[/math] where d is the distance between parallel sides, or the height when the decagon stands on one side as base.
By simple trigonometry [math]\displaystyle{ d = 2t(\cos{54^\circ} + \cos{18^\circ}) }[/math].

Sides

The side of a regular decagon inscribed in a unit circle is [math]\displaystyle{ \tfrac{-1+\sqrt{5}}{2}=\tfrac{1}{\phi} }[/math], where ϕ is the golden ratio, [math]\displaystyle{ \tfrac{1+\sqrt{5}}{2} }[/math].

Dissection of regular decagon

Coxeter states that every parallel-sided 2m-gon can be divided into m(m-1)/2 rhombs. For the regular decagon, m=5, and it can be divided into 10 rhombs, with examples shown below. This decomposition can be seen as 10 of 80 faces in a Petrie polygon projection plane of the 5-cube. A dissection is based on 10 of 30 faces of the rhombic triacontahedron.^[1] The list A006245 defines the number of solutions as 62, with 2 orientations for the first symmetric form, and 10 orientations for the other 6.

Regular decagon dissected into 10 rhombi
5-cube

Skew decagon

3 regular skew zig-zag decagons
{5}#{ }	{5/2}#{ }	{5/3}#{ }

A regular skew decagon is seen as zig-zagging edges of a pentagonal antiprism, a pentagrammic antiprism, and a pentagrammic crossed-antiprism.

A skew decagon is a skew polygon with 10 vertices and edges but not existing on the same plane. The interior of such an decagon is not generally defined. A skew zig-zag decagon has vertices alternating between two parallel planes.

A regular skew decagon is vertex-transitive with equal edge lengths. In 3-dimensions it will be a zig-zag skew decagon and can be seen in the vertices and side edges of a pentagonal antiprism, pentagrammic antiprism, and pentagrammic crossed-antiprism with the same D_5d, [2⁺,10] symmetry, order 20.

These can also be seen in these 4 convex polyhedra with icosahedral symmetry. The polygons on the perimeter of these projections are regular skew decagons.

Orthogonal projections of polyhedra on 5-fold axes
Dodecahedron	Icosahedron	Icosidodecahedron	Rhombic triacontahedron

Decagon Media

Decagon at a given side length
Symmetries of a regular decagon. Vertices are colored by their symmetry positions. Blue mirrors are drawn through vertices, and purple mirrors are drawn through edges. Gyration orders are given in the center.
en:10-cube, A9 projection
10-gon rhombic dissection-size2
10-gon rhombic dissection2-size2
10-gon rhombic dissection3-size2
10-gon rhombic dissection4-size2
10-gon rhombic dissection5-size2
10-gon rhombic dissection6-size2
10-gon rhombic dissection7-size2