Regular dodecagon | |
---|---|

Type | Regular polygon |

Edges and vertices | 12 |

Schläfli symbol | {12}, t{6}, tt{3} |

Coxeter–Dynkin diagrams | |

Symmetry group | Dihedral (D_{12}), order 2×12 |

Internal angle (degrees) | 150° |

Properties | Convex, cyclic, equilateral, isogonal, isotoxal |

In geometry, a **dodecagon** or 12-gon is any twelve-sided polygon.

A regular dodecagon is a figure with sides of the same length and internal angles of the same size. It has twelve lines of reflective symmetry and rotational symmetry of order 12. A regular dodecagon is represented by the Schläfli symbol {12} and can be constructed as a truncated hexagon, t{6}, or a twice-truncated triangle, tt{3}. The internal angle at each vertex of a regular dodecagon is 150°.

The area of a regular dodecagon of side length *a* is given by:

And in terms of the apothem *r* (see also inscribed figure), the area is:

In terms of the circumradius *R*, the area is:^{ [1] }

The span *S* of the dodecagon is the distance between two parallel sides and is equal to twice the apothem. A simple formula for area (given side length and span) is:

This can be verified with the trigonometric relationship:

The perimeter of a regular dodecagon in terms of circumradius is:^{ [2] }

The perimeter in terms of apothem is:

This coefficient is double the coefficient found in the apothem equation for area.^{ [3] }

As 12 = 2^{2} × 3, regular dodecagon is constructible using compass-and-straightedge construction:

12-cube | 60 rhomb dissection | |||
---|---|---|---|---|

Coxeter states that every zonogon (a 2*m*-gon whose opposite sides are parallel and of equal length) can be dissected into *m*(*m*-1)/2 parallelograms.^{ [4] } In particular this is true for regular polygons with evenly many sides, in which case the parallelograms are all rhombi. For the *regular dodecagon*, *m*=6, and it can be divided into 15: 3 squares, 6 wide 30° rhombs and 6 narrow 15° rhombs. This decomposition is based on a Petrie polygon projection of a 6-cube, with 15 of 240 faces. The sequence OEIS sequence A006245 defines the number of solutions as 908, including up to 12-fold rotations and chiral forms in reflection.

6-cube | |||||

One of the ways the mathematical manipulative pattern blocks are used is in creating a number of different dodecagons.^{ [5] } They are related to the rhombic dissections, with 3 60° rhombi merged into hexagons, half-hexagon trapezoids, or divided into 2 equilateral triangles.

Socolar tiling | Pattern blocks |

The *regular dodecagon* has Dih_{12} symmetry, order 24. There are 15 distinct subgroup dihedral and cyclic symmetries. Each subgroup symmetry allows one or more degrees of freedom for irregular forms. Only the **g12** subgroup has no degrees of freedom but can seen as directed edges.

Example dodecagons by symmetry | ||||||
---|---|---|---|---|---|---|

r24 | ||||||

d12 | g12 | p12 | i8 | |||

d6 | g6 | p6 | d4 | g4 | p4 | |

g3 | d2 | g2 | p2 | |||

a1 |

A regular dodecagon can fill a plane vertex with other regular polygons in 4 ways:

3.12.12 | 4.6.12 | 3.3.4.12 | 3.4.3.12 |
---|

Here are 3 example periodic plane tilings that use regular dodecagons, defined by their vertex configuration:

1-uniform | 2-uniform | |
---|---|---|

3.12.12 | 4.6.12 | 3.12.12; 3.4.3.12 |

A **skew dodecagon** is a skew polygon with 12 vertices and edges but not existing on the same plane. The interior of such an dodecagon is not generally defined. A *skew zig-zag dodecagon* has vertices alternating between two parallel planes.

A ** regular skew dodecagon ** is vertex-transitive with equal edge lengths. In 3-dimensions it will be a zig-zag skew dodecagon and can be seen in the vertices and side edges of a hexagonal antiprism with the same D_{5d}, [2^{+},10] symmetry, order 20. The dodecagrammic antiprism, s{2,24/5} and dodecagrammic crossed-antiprism, s{2,24/7} also have regular skew dodecagons.

The regular dodecagon is the Petrie polygon for many higher-dimensional polytopes, seen as orthogonal projections in Coxeter planes. Examples in 4 dimensions are the 24-cell, snub 24-cell, 6-6 duoprism, 6-6 duopyramid. In 6 dimensions 6-cube, 6-orthoplex, 2_{21}, 1_{22}. It is also the Petrie polygon for the grand 120-cell and great stellated 120-cell.

Regular skew dodecagons in higher dimensions | |||||
---|---|---|---|---|---|

E_{6} | F_{4} | 2G_{2} (4D) | |||

2 _{21} | 1 _{22} | 24-cell | Snub 24-cell | 6-6 duopyramid | {6}×{6} |

A_{11} | D_{7} | B_{6} | 4A_{2} | ||

11-simplex | (4 _{11}) | 1 _{41} | 6-orthoplex | 6-cube | {3}×{3}×{3}×{3} |

A dodecagram is a 12-sided star polygon, represented by symbol {12/n}. There is one regular star polygon: {12/5}, using the same vertices, but connecting every fifth point. There are also three compounds: {12/2} is reduced to 2{6} as two hexagons, and {12/3} is reduced to 3{4} as three squares, {12/4} is reduced to 4{3} as four triangles, and {12/6} is reduced to 6{2} as six degenerate digons.

Stars and compounds | ||||||
---|---|---|---|---|---|---|

n | 1 | 2 | 3 | 4 | 5 | 6 |

Form | Polygon | Compounds | Star polygon | Compound | ||

Image | {12/1} = {12} | {12/2} or 2{6} | {12/3} or 3{4} | {12/4} or 4{3} | {12/5} | {12/6} or 6{2} |

Deeper truncations of the regular dodecagon and dodecagrams can produce isogonal (vertex-transitive) intermediate star polygon forms with equal spaced vertices and two edge lengths. A truncated hexagon is a dodecagon, t{6}={12}. A quasitruncated hexagon, inverted as {6/5}, is a dodecagram: t{6/5}={12/5}.^{ [7] }

Vertex-transitive truncations of the hexagon | |||
---|---|---|---|

Quasiregular | Isogonal | Quasiregular | |

t{6}={12} | t{6/5}={12/5} |

In block capitals, the letters E, H and X (and I in a slab serif font) have dodecagonal outlines. A cross is a dodecagon, as is the logo for the Chevrolet automobile division.

The regular dodecagon features prominently in many buildings. The Torre del Oro is a dodecagonal military watchtower in Seville, southern Spain, built by the Almohad dynasty. The early thirteenth century Vera Cruz church in Segovia, Spain is dodecagonal. Another example is the Porta di Venere (Venus' Gate), in Spello, Italy, built in the 1st century BC has two dodecagonal towers, called "Propertius' Towers".

Regular dodecagonal coins include:

- British threepenny bit from 1937 to 1971, when it ceased to be legal tender.
- British One Pound Coin, introduced in 2017.
- Australian 50-cent coin
- Fijian 50 cents
- Tongan 50-seniti, since 1974
- Solomon Islands 50 cents
- Croatian 25 kuna
- Romanian 5000 lei, 2001–2005
- Canadian penny, 1982–1996
- South Vietnamese 20 đồng, 1968–1975
- Zambian 50 ngwee, 1969–1992
- Malawian 50 tambala, 1986–1995
- Mexican 20 centavos, 1992-2009

- Dodecagonal number
- Dodecahedron – any polyhedron with 12 faces.
- Dodecagram

- ↑ See also Kürschák's geometric proof on the Wolfram Demonstration Project
- ↑
*Plane Geometry: Experiment, Classification, Discovery, Application*by Clarence Addison Willis B., (1922) Blakiston's Son & Company, p. 249 - ↑
*Elements of geometry*by John Playfair, William Wallace, John Davidsons, (1814) Bell & Bradfute, p. 243 - ↑ Coxeter, Mathematical recreations and Essays, Thirteenth edition, p.141
- ↑ "Doin' Da' Dodeca'" on mathforum.org
- ↑ John H. Conway, Heidi Burgiel, Chaim Goodman-Strauss, (2008) The Symmetries of Things, ISBN 978-1-56881-220-5 (Chapter 20, Generalized Schaefli symbols, Types of symmetry of a polygon pp. 275-278)
- ↑ The Lighter Side of Mathematics: Proceedings of the Eugène Strens Memorial Conference on Recreational Mathematics and its History, (1994),
*Metamorphoses of polygons*, Branko Grünbaum

- Weisstein, Eric W. "Dodecagon".
*MathWorld*. - Kürschak's Tile and Theorem
- Definition and properties of a dodecagon With interactive animation
- The regular dodecagon in the classroom, using pattern blocks

In geometry, an **n-gonal antiprism** or **n-antiprism** is a polyhedron composed of two parallel direct copies of an n-sided polygon, connected by an alternating band of 2*n* triangles. They are represented by the Conway notation A*n*.

A (symmetric) n-gonal **bipyramid** or **dipyramid** is a polyhedron formed by joining an n-gonal pyramid and its mirror image base-to-base. An n-gonal bipyramid has 2*n* triangle faces, 3*n* edges, and 2 + *n* vertices.

In geometry, a **polygon** is a plane figure that is described by a finite number of straight line segments connected to form a closed *polygonal chain*. The bounded plane region, the bounding circuit, or the two together, may be called a polygon.

In geometry, a **Platonic solid** is a convex, regular polyhedron in three-dimensional Euclidean space. Being a regular polyhedron means that the faces are congruent regular polygons, and the same number of faces meet at each vertex. There are only five such polyhedra:

In geometry, a **hexagon** is a six-sided polygon or 6-gon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°.

In geometry, an **octagon** is an eight-sided polygon or 8-gon.

In geometry, a **decagon** is a ten-sided polygon or 10-gon. The total sum of the interior angles of a simple decagon is 1440°.

In Euclidean geometry, a **regular polygon** is a polygon that is direct equiangular and equilateral. Regular polygons may be either **convex**, **star** or **skew**. In the limit, a sequence of regular polygons with an increasing number of sides approximates a circle, if the perimeter or area is fixed, or a regular apeirogon, if the edge length is fixed.

In geometry, a **heptagon** or **septagon** is a seven-sided polygon or 7-gon.

In geometry, a **nonagon** or **enneagon** is a nine-sided polygon or 9-gon.

In geometry, a **triacontagon** or 30-gon is a thirty-sided polygon. The sum of any triacontagon's interior angles is 5040 degrees.

In geometry, the **snub disphenoid**, **Siamese dodecahedron**, **triangular dodecahedron**, **trigonal dodecahedron**, or **dodecadeltahedron** is a convex polyhedron with twelve equilateral triangles as its faces. It is not a regular polyhedron because some vertices have four faces and others have five. It is a dodecahedron, one of the eight deltahedra, and is the 84th Johnson solid. It can be thought of as a square antiprism where both squares are replaced with two equilateral triangles.

In geometry, a **pentadecagon** or **pentakaidecagon** or 15-gon is a fifteen-sided polygon.

In geometry, a **pyramid** is a polyhedron formed by connecting a polygonal base and a point, called the apex. Each base edge and apex form a triangle, called a *lateral face*. It is a conic solid with polygonal base. A pyramid with an n-sided base has *n* + 1 vertices, *n* + 1 faces, and 2*n* edges. All pyramids are self-dual.

In mathematics, a **hexadecagon** is a sixteen-sided polygon.

In geometry, a **pentagon** is any five-sided polygon or 5-gon. The sum of the internal angles in a simple pentagon is 540°.

In geometry, an **icositetragon** or 24-gon is a twenty-four-sided polygon. The sum of any icositetragon's interior angles is 3960 degrees.

In geometry, a **dodecagram** is a star polygon or compound with 12 vertices. There is one regular dodecagram polygon. There are also 4 regular compounds {12/2},{12/3},{12/4}, and {12/6}.

In geometry, a **planigon** is a convex polygon that can fill the plane with only copies of itself. In the Euclidean plane there are 3 regular planigons; equilateral triangle, squares, and regular hexagons; and 8 semiregular planigons; and 4 demiregular planigons which can tile the plane only with other planigons.

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Images, videos and audio are available under their respective licenses.