Hexagon

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Short description: Shape with six sides
Regular hexagon
Regular polygon 6 annotated.svg
A regular hexagon
TypeRegular polygon
Edges and vertices6
Schläfli symbol{6}, t{3}
Coxeter diagramCDel node 1.pngCDel 6.pngCDel node.png
CDel node 1.pngCDel 3.pngCDel node 1.png
Symmetry groupDihedral (D6), order 2×6
Internal angle (degrees)120°
Dual polygonSelf
PropertiesConvex, cyclic, equilateral, isogonal, isotoxal

In geometry, a hexagon (from Greek ἕξ, hex, meaning "six", and γωνία, gonía, meaning "corner, angle") is a six-sided polygon.[1] The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°.

Regular hexagon

A regular hexagon has Schläfli symbol {6}[2] and can also be constructed as a truncated equilateral triangle, t{3}, which alternates two types of edges.

A regular hexagon is defined as a hexagon that is both equilateral and equiangular. It is bicentric, meaning that it is both cyclic (has a circumscribed circle) and tangential (has an inscribed circle).

The common length of the sides equals the radius of the circumscribed circle or circumcircle, which equals [math]\displaystyle{ \tfrac{2}{\sqrt{3}} }[/math] times the apothem (radius of the inscribed circle). All internal angles are 120 degrees. A regular hexagon has six rotational symmetries (rotational symmetry of order six) and six reflection symmetries (six lines of symmetry), making up the dihedral group D6. The longest diagonals of a regular hexagon, connecting diametrically opposite vertices, are twice the length of one side. From this it can be seen that a triangle with a vertex at the center of the regular hexagon and sharing one side with the hexagon is equilateral, and that the regular hexagon can be partitioned into six equilateral triangles.

Like squares and equilateral triangles, regular hexagons fit together without any gaps to tile the plane (three hexagons meeting at every vertex), and so are useful for constructing tessellations. The cells of a beehive honeycomb are hexagonal for this reason and because the shape makes efficient use of space and building materials. The Voronoi diagram of a regular triangular lattice is the honeycomb tessellation of hexagons. It is not usually considered a triambus, although it is equilateral.

A step-by-step animation of the construction of a regular hexagon using compass and straightedge, given by Euclid's Elements, Book IV, Proposition 15: this is possible as 6 [math]\displaystyle{ = }[/math] 2 × 3, a product of a power of two and distinct Fermat primes.
When the side length AB is given, drawing a circular arc from point A and point B gives the intersection M, the center of the circumscribed circle. Transfer the line segment AB four times on the circumscribed circle and connect the corner points.

Parameters

R = Circumradius; r = Inradius; t = side length

The maximal diameter (which corresponds to the long diagonal of the hexagon), D, is twice the maximal radius or circumradius, R, which equals the side length, t. The minimal diameter or the diameter of the inscribed circle (separation of parallel sides, flat-to-flat distance, short diagonal or height when resting on a flat base), d, is twice the minimal radius or inradius, r. The maxima and minima are related by the same factor:

[math]\displaystyle{ \frac{1}{2}d = r = \cos(30^\circ) R = \frac{\sqrt{3}}{2} R = \frac{\sqrt{3}}{2} t }[/math]   and, similarly, [math]\displaystyle{ d = \frac{\sqrt{3}}{2} D. }[/math]

The area of a regular hexagon

[math]\displaystyle{ \begin{align} A &= \frac{3\sqrt{3}}{2}R^2 = 3Rr = 2\sqrt{3} r^2 \\[3pt] &= \frac{3\sqrt{3}}{8}D^2 = \frac{3}{4}Dd = \frac{\sqrt{3}}{2} d^2 \\[3pt] &\approx 2.598 R^2 \approx 3.464 r^2\\ &\approx 0.6495 D^2 \approx 0.866 d^2. \end{align} }[/math]

For any regular polygon, the area can also be expressed in terms of the apothem a and the perimeter p. For the regular hexagon these are given by a = r, and p[math]\displaystyle{ {} = 6R = 4r\sqrt{3} }[/math], so

[math]\displaystyle{ \begin{align} A &= \frac{ap}{2} \\ &= \frac{r \cdot 4r\sqrt{3}}{2} = 2r^2\sqrt{3} \\ &\approx 3.464 r^2. \end{align} }[/math]

The regular hexagon fills the fraction [math]\displaystyle{ \tfrac{3\sqrt{3}}{2\pi} \approx 0.8270 }[/math] of its circumscribed circle.

If a regular hexagon has successive vertices A, B, C, D, E, F and if P is any point on the circumcircle between B and C, then PE + PF = PA + PB + PC + PD.

It follows from the ratio of circumradius to inradius that the height-to-width ratio of a regular hexagon is 1:1.1547005; that is, a hexagon with a long diagonal of 1.0000000 will have a distance of 0.8660254 between parallel sides.

Point in plane

For an arbitrary point in the plane of a regular hexagon with circumradius [math]\displaystyle{ R }[/math], whose distances to the centroid of the regular hexagon and its six vertices are [math]\displaystyle{ L }[/math] and [math]\displaystyle{ d_i }[/math] respectively, we have[3]

[math]\displaystyle{ d_1^2 + d_4^2 = d_2^2 + d_5^2 = d_3^2+ d_6^2= 2\left(R^2 + L^2\right), }[/math]
[math]\displaystyle{ d_1^2 + d_3^2+ d_5^2 = d_2^2 + d_4^2+ d_6^2 = 3\left(R^2 + L^2\right), }[/math]
[math]\displaystyle{ d_1^4 + d_3^4+ d_5^4 = d_2^4 + d_4^4+ d_6^4 = 3\left(\left(R^2 + L^2\right)^2 + 2 R^2 L^2\right). }[/math]

If [math]\displaystyle{ d_i }[/math] are the distances from the vertices of a regular hexagon to any point on its circumcircle, then [3]

[math]\displaystyle{ \left(\sum_{i=1}^6 d_i^2\right)^2 = 4 \sum_{i=1}^6 d_i^4 . }[/math]

Symmetry

The six lines of reflection of a regular hexagon, with Dih6 or r12 symmetry, order 12.
The dihedral symmetries are divided depending on whether they pass through vertices (d for diagonal) or edges (p for perpendiculars) Cyclic symmetries in the middle column are labeled as g for their central gyration orders. Full symmetry of the regular form is r12 and no symmetry is labeled a1.

The regular hexagon has D6 symmetry. There are 16 subgroups. There are 8 up to isomorphism: itself (D6), 2 dihedral: (D3, D2), 4 cyclic: (Z6, Z3, Z2, Z1) and the trivial (e)

These symmetries express nine distinct symmetries of a regular hexagon. John Conway labels these by a letter and group order.[4] r12 is full symmetry, and a1 is no symmetry. p6, an isogonal hexagon constructed by three mirrors can alternate long and short edges, and d6, an isotoxal hexagon 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 hexagon. The i4 forms are regular hexagons flattened or stretched along one symmetry direction. It can be seen as an elongated rhombus, while d2 and p2 can be seen as horizontally and vertically elongated kites. g2 hexagons, with opposite sides parallel are also called hexagonal parallelogons.

Each subgroup symmetry allows one or more degrees of freedom for irregular forms. Only the g6 subgroup has no degrees of freedom but can be seen as directed edges.

Hexagons of symmetry g2, i4, and r12, as parallelogons can tessellate the Euclidean plane by translation. Other hexagon shapes can tile the plane with different orientations.

p6m (*632) cmm (2*22) p2 (2222) p31m (3*3) pmg (22*) pg (××)
Isohedral tiling p6-13.png
r12
Isohedral tiling p6-12.png
i4
Isohedral tiling p6-7.png
g2
Isohedral tiling p6-11.png
d2
Isohedral tiling p6-10.png
d2
Isohedral tiling p6-9.png
p2
Isohedral tiling p6-1.png
a1
Dih6 Dih2 Z2 Dih1 Z1

A2 and G2 groups

Root system A2.svg
A2 group roots
Script error: No such module "Dynkin".
Root system G2.svg
G2 group roots
Dyn2-nodeg n1.pngDyn2-6a.pngDyn2-node n2.png

The 6 roots of the simple Lie group A2, represented by a Dynkin diagram Script error: No such module "Dynkin"., are in a regular hexagonal pattern. The two simple roots have a 120° angle between them.

The 12 roots of the Exceptional Lie group G2, represented by a Dynkin diagram Dyn2-nodeg n1.pngDyn2-6a.pngDyn2-node n2.png are also in a hexagonal pattern. The two simple roots of two lengths have a 150° angle between them.

Dissection

6-cube projection 12 rhomb dissection
6-cube t0 A5.svg 6-gon rhombic dissection-size2.svg 6-gon rhombic dissection2-size2.svg

Coxeter states that every zonogon (a 2m-gon whose opposite sides are parallel and of equal length) can be dissected into 12m(m − 1) parallelograms.[5] In particular this is true for regular polygons with evenly many sides, in which case the parallelograms are all rhombi. This decomposition of a regular hexagon is based on a Petrie polygon projection of a cube, with 3 of 6 square faces. Other parallelogons and projective directions of the cube are dissected within rectangular cuboids.

Dissection of hexagons into three rhombs and parallelograms
2D Rhombs Parallelograms
Hexagon dissection.svg Cube-skew-orthogonal-skew-solid.png Cuboid diagonal-orthogonal-solid.png Cuboid skew-orthogonal-solid.png
Regular {6} Hexagonal parallelogons
3D Square faces Rectangular faces
3-cube graph.svg Cube-skew-orthogonal-skew-frame.png Cuboid diagonal-orthogonal-frame.png Cuboid skew-orthogonal-frame.png
Cube Rectangular cuboid

Related polygons and tilings

A regular hexagon has Schläfli symbol {6}. A regular hexagon is a part of the regular hexagonal tiling, {6,3}, with three hexagonal faces around each vertex.

A regular hexagon can also be created as a truncated equilateral triangle, with Schläfli symbol t{3}. Seen with two types (colors) of edges, this form only has D3 symmetry.

A truncated hexagon, t{6}, is a dodecagon, {12}, alternating two types (colors) of edges. An alternated hexagon, h{6}, is an equilateral triangle, {3}. A regular hexagon can be stellated with equilateral triangles on its edges, creating a hexagram. A regular hexagon can be dissected into six equilateral triangles by adding a center point. This pattern repeats within the regular triangular tiling.

A regular hexagon can be extended into a regular dodecagon by adding alternating squares and equilateral triangles around it. This pattern repeats within the rhombitrihexagonal tiling.

Regular polygon 6 annotated.svg Truncated triangle.svg Regular truncation 3 1000.svg Regular truncation 3 1.5.svg Regular truncation 3 0.55.svg Hexagram.svg Regular polygon 12 annotated.svg Regular polygon 3 annotated.svg
Regular
{6}
Truncated
t{3} = {6}
Hypertruncated triangles Stellated
Star figure 2{3}
Truncated
t{6} = {12}
Alternated
h{6} = {3}
Crossed-square hexagon.png Medial triambic icosahedron face.svg Great triambic icosahedron face.png Hexagonal cupola flat.svg Cube petrie polygon sideview.svg 3-cube t0.svg 3-cube t2.svg 5-simplex graph.svg
Crossed
hexagon
A concave hexagon A self-intersecting hexagon (star polygon) Extended
Central {6} in {12}
A skew hexagon, within cube Dissected {6} projection
octahedron
Complete graph

Self-crossing hexagons

There are six self-crossing hexagons with the vertex arrangement of the regular hexagon:

Self-intersecting hexagons with regular vertices
Dih2 Dih1 Dih3
Crossed hexagon1.svg
Figure-eight
Crossed hexagon2.svg
Center-flip
Crossed hexagon3.svg
Unicursal
Crossed hexagon4.svg
Fish-tail
Crossed hexagon5.svg
Double-tail
Crossed hexagon6.svg
Triple-tail

Hexagonal structures

Giant's Causeway closeup

From bees' honeycombs to the Giant's Causeway, hexagonal patterns are prevalent in nature due to their efficiency. In a hexagonal grid each line is as short as it can possibly be if a large area is to be filled with the fewest hexagons. This means that honeycombs require less wax to construct and gain much strength under compression.

Irregular hexagons with parallel opposite edges are called parallelogons and can also tile the plane by translation. In three dimensions, hexagonal prisms with parallel opposite faces are called parallelohedrons and these can tessellate 3-space by translation.

Hexagonal prism tessellations
Form Hexagonal tiling Hexagonal prismatic honeycomb
Regular Uniform tiling 63-t0.png Hexagonal prismatic honeycomb.png
Parallelogonal Isohedral tiling p6-7.png Skew hexagonal prism honeycomb.png

Tesselations by hexagons

Main page: Hexagonal tiling

In addition to the regular hexagon, which determines a unique tessellation of the plane, any irregular hexagon which satisfies the Conway criterion will tile the plane.

Hexagon inscribed in a conic section

Pascal's theorem (also known as the "Hexagrammum Mysticum Theorem") states that if an arbitrary hexagon is inscribed in any conic section, and pairs of opposite sides are extended until they meet, the three intersection points will lie on a straight line, the "Pascal line" of that configuration.

Cyclic hexagon

The Lemoine hexagon is a cyclic hexagon (one inscribed in a circle) with vertices given by the six intersections of the edges of a triangle and the three lines that are parallel to the edges that pass through its symmedian point.

If the successive sides of a cyclic hexagon are a, b, c, d, e, f, then the three main diagonals intersect in a single point if and only if ace = bdf.[6]

If, for each side of a cyclic hexagon, the adjacent sides are extended to their intersection, forming a triangle exterior to the given side, then the segments connecting the circumcenters of opposite triangles are concurrent.[7]

If a hexagon has vertices on the circumcircle of an acute triangle at the six points (including three triangle vertices) where the extended altitudes of the triangle meet the circumcircle, then the area of the hexagon is twice the area of the triangle.[8]:p. 179

Hexagon tangential to a conic section

Let ABCDEF be a hexagon formed by six tangent lines of a conic section. Then Brianchon's theorem states that the three main diagonals AD, BE, and CF intersect at a single point.

In a hexagon that is tangential to a circle and that has consecutive sides a, b, c, d, e, and f,[9]

[math]\displaystyle{ a + c + e = b + d + f. }[/math]

Equilateral triangles on the sides of an arbitrary hexagon

Equilateral triangles on the sides of an arbitrary hexagon

If an equilateral triangle is constructed externally on each side of any hexagon, then the midpoints of the segments connecting the centroids of opposite triangles form another equilateral triangle.[10]:Thm. 1

Skew hexagon

A regular skew hexagon seen as edges (black) of a triangular antiprism, symmetry D3d, [2+,6], (2*3), order 12.

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

A regular skew hexagon is vertex-transitive with equal edge lengths. In three dimensions it will be a zig-zag skew hexagon and can be seen in the vertices and side edges of a triangular antiprism with the same D3d, [2+,6] symmetry, order 12.

The cube and octahedron (same as triangular antiprism) have regular skew hexagons as petrie polygons.

Skew hexagons on 3-fold axes
Cube petrie.png
Cube
Octahedron petrie.png
Octahedron

Petrie polygons

The regular skew hexagon is the Petrie polygon for these higher dimensional regular, uniform and dual polyhedra and polytopes, shown in these skew orthogonal projections:

4D 5D
3-3 duoprism ortho-Dih3.png
3-3 duoprism
3-3 duopyramid ortho.png
3-3 duopyramid
5-simplex t0.svg
5-simplex

Convex equilateral hexagon

A principal diagonal of a hexagon is a diagonal which divides the hexagon into quadrilaterals. In any convex equilateral hexagon (one with all sides equal) with common side a, there exists[11]:p.184,#286.3 a principal diagonal d1 such that

[math]\displaystyle{ \frac{d_1}{a} \leq 2 }[/math]

and a principal diagonal d2 such that

[math]\displaystyle{ \frac{d_2}{a} \gt \sqrt{3}. }[/math]

Polyhedra with hexagons

There is no Platonic solid made of only regular hexagons, because the hexagons tessellate, not allowing the result to "fold up". The Archimedean solids with some hexagonal faces are the truncated tetrahedron, truncated octahedron, truncated icosahedron (of soccer ball and fullerene fame), truncated cuboctahedron and the truncated icosidodecahedron. These hexagons can be considered truncated triangles, with Coxeter diagrams of the form CDel node 1.pngCDel 3.pngCDel node 1.pngCDel p.pngCDel node.png and CDel node 1.pngCDel 3.pngCDel node 1.pngCDel p.pngCDel node 1.png.

There are other symmetry polyhedra with stretched or flattened hexagons, like these Goldberg polyhedron G(2,0):

There are also 9 Johnson solids with regular hexagons:

Gallery of natural and artificial hexagons

See also

References

  1. Cube picture
  2. Wenninger, Magnus J. (1974), Polyhedron Models, Cambridge University Press, p. 9, ISBN 9780521098595, archived from the original. Error: If you specify |archiveurl=, you must also specify |archivedate=, https://web.archive.org/web/20160102075753/https://books.google.com/books?id=N8lX2T-4njIC&pg=PA9, retrieved 2015-11-06 .
  3. 3.0 3.1 Meskhishvili, Mamuka (2020). "Cyclic Averages of Regular Polygons and Platonic Solids". Communications in Mathematics and Applications 11: 335–355. doi:10.26713/cma.v11i3.1420. https://www.rgnpublications.com/journals/index.php/cma/article/view/1420/1065. 
  4. 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)
  5. Coxeter, Mathematical recreations and Essays, Thirteenth edition, p.141
  6. Cartensen, Jens, "About hexagons", Mathematical Spectrum 33(2) (2000–2001), 37–40.
  7. Dergiades, Nikolaos (2014). "Dao's theorem on six circumcenters associated with a cyclic hexagon". Forum Geometricorum 14: 243–246. http://forumgeom.fau.edu/FG2014volume14/FG201424index.html. Retrieved 2014-11-17. 
  8. Johnson, Roger A., Advanced Euclidean Geometry, Dover Publications, 2007 (orig. 1960).
  9. Gutierrez, Antonio, "Hexagon, Inscribed Circle, Tangent, Semiperimeter", [1] , Accessed 2012-04-17.
  10. Dao Thanh Oai (2015). "Equilateral triangles and Kiepert perspectors in complex numbers". Forum Geometricorum 15: 105–114. http://forumgeom.fau.edu/FG2015volume15/FG201509index.html. Retrieved 2015-04-12. 
  11. Inequalities proposed in "Crux Mathematicorum", [2] .

External links