Decagon
Regular decagon  

A regular decagon  
Type  Regular polygon 
Edges and vertices  10 
Schläfli symbol  {10}, t{5} 
Coxeter diagram 

Symmetry group  Dihedral (D_{10}), order 2×10 
Internal angle (degrees)  144° 
Dual polygon  Self 
Properties  Convex, cyclic, equilateral, isogonal, isotoxal 
In geometry, a decagon is a tensided polygon or 10gon.^{[1]}
Regular decagon
A regular decagon has all sides of equal length and each internal angle will always be equal to 144°.^{[1]} Its Schläfli symbol is {10} ^{[2]} and can also be constructed as a truncated pentagon, t{5}, a quasiregular decagon alternating two types of edges.
Area
The area of a regular decagon of side length a is given by:^{[3]}
In terms of the apothem r (see also inscribed figure), the area is:
In terms of the circumradius R, the area is:
An alternative formula is
where d is the distance between parallel sides, or the height when the decagon stands on one side as base, or the diameter of the decagon's inscribed circle.
By simple trigonometry,
and it can be written algebraically as
Sides
The side of a regular decagon inscribed in a unit circle is , where ϕ is the golden ratio, .^{[4]}
Construction
As 10 = 2 × 5, a power of two times a Fermat prime, it follows that a regular decagon is constructible using compass and straightedge, or by an edgebisection of a regular pentagon.^{[4]}
An alternative (but similar) method is as follows:
 Construct a pentagon in a circle by one of the methods shown in constructing a pentagon.
 Extend a line from each vertex of the pentagon through the center of the circle to the opposite side of that same circle. Where each line cuts the circle is a vertex of the decagon.
 The five corners of the pentagon constitute alternate corners of the decagon. Join these points to the adjacent new points to form the decagon.
Nonconvex regular decagon
The length ratio of two inequal edges of a golden triangle is the golden ratio, denoted or its multiplicative inverse:
So we can get the properties of a regular decagonal star, through a tiling by golden triangles that fills this star polygon.
The golden ratio in decagon
Both in the construction with given circumcircle^{[5]} as well as with given side length is the golden ratio dividing a line segment by exterior division the determining construction element.
 In the construction with given circumcircle the circular arc around G with radius GE_{3} produces the segment AH, whose division corresponds to the golden ratio.
 In the construction with given side length^{[6]} the circular arc around D with radius DA produces the segment E_{10}F, whose division corresponds to the golden ratio.
Symmetry
The regular decagon has Dih_{10} symmetry, order 20. There are 3 subgroup dihedral symmetries: Dih_{5}, Dih_{2}, and Dih_{1}, and 4 cyclic group symmetries: Z_{10}, Z_{5}, Z_{2}, and Z_{1}.
These 8 symmetries can be seen in 10 distinct symmetries on the decagon, a larger number because the lines of reflections can either pass through vertices or edges. John Conway labels these by a letter and group order.^{[7]} Full symmetry of the regular form is r20 and no symmetry is labeled a1. The dihedral symmetries are divided depending on whether they pass through vertices (d for diagonal) or edges (p for perpendiculars), and i when reflection lines path through both edges and vertices. Cyclic symmetries in the middle column are labeled as g for their central gyration orders.
Each subgroup symmetry allows one or more degrees of freedom for irregular forms. Only the g10 subgroup has no degrees of freedom but can seen as directed edges.
The highest symmetry irregular decagons are d10, a isogonal decagon constructed by five mirrors which can alternate long and short edges, and p10, an isotoxal decagon, constructed with equal edge lengths, but vertices alternating two different internal angles. These two forms are duals of each other and have half the symmetry order of the regular decagon.
Dissection
10cube projection  40 rhomb dissection  

Coxeter states that every zonogon (a 2mgon whose opposite sides are parallel and of equal length) can be dissected into m(m1)/2 parallelograms.^{[8]}
In particular this is true for regular polygons with evenly many sides, in which case the parallelograms are all rhombi. 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 5cube. A dissection is based on 10 of 30 faces of the rhombic triacontahedron. The list
5cube 

Skew decagon
{5}#{ }  {5/2}#{ }  {5/3}#{ } 

A regular skew decagon is seen as zigzagging edges of a pentagonal antiprism, a pentagrammic antiprism, and a pentagrammic crossedantiprism. 
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 zigzag decagon has vertices alternating between two parallel planes.
A regular skew decagon is vertextransitive with equal edge lengths. In 3dimensions it will be a zigzag skew decagon and can be seen in the vertices and side edges of a pentagonal antiprism, pentagrammic antiprism, and pentagrammic crossedantiprism 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.
Dodecahedron 
Icosahedron 
Icosidodecahedron 
Rhombic triacontahedron 
Petrie polygons
The regular skew decagon is the Petrie polygon for many higherdimensional polytopes, shown in these orthogonal projections in various Coxeter planes:^{[9]} The number of sides in the Petrie polygon is equal to the Coxeter number, h, for each symmetry family.
A_{9}  D_{6}  B_{5}  

9simplex 
4_{11} 
1_{31} 
5orthoplex 
5cube 
See also
 Decagonal number and centered decagonal number, figurate numbers modeled on the decagon
 Decagram, a star polygon with the same vertex positions as the regular decagon
References
 1 2 Sidebotham, Thomas H. (2003), The A to Z of Mathematics: A Basic Guide, John Wiley & Sons, p. 146, ISBN 9780471461630 .
 ↑ Wenninger, Magnus J. (1974), Polyhedron Models, Cambridge University Press, p. 9, ISBN 9780521098595 .
 ↑ The elements of plane and spherical trigonometry, Society for Promoting Christian Knowledge, 1850, p. 59 . 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.
 1 2 Ludlow, Henry H. (1904), Geometric Construction of the Regular Decagon and Pentagon Inscribed in a Circle, The Open Court Publishing Co. .
 1 2 Green, Henry (1861), Euclid's Plane Geometry, Books III–VI, Practically Applied, or Gradations in Euclid, Part II, London: Simpkin, Marshall,& CO., p. 116 . Retrieved 10 February 2016.
 1 2 (in German) Köller, Jürgen (2005), Regelmäßiges Zehneck, → 3. Section "Formeln, Ist die Seite a gegeben ..." . Retrieved 10 February 2016.
 ↑ John H. Conway, Heidi Burgiel, Chaim GoodmanStrauss, (2008) The Symmetries of Things, ISBN 9781568812205 (Chapter 20, Generalized Schaefli symbols, Types of symmetry of a polygon pp. 275278)
 ↑ Coxeter, Mathematical recreations and Essays, Thirteenth edition, p.141
 ↑ Coxeter, Regular polytopes, 12.4 Petrie polygon, pp. 223226.
External links
 Weisstein, Eric W. "Decagon". MathWorld.
 Definition and properties of a decagon With interactive animation