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A regular heptagon
|Edges and vertices||7|
|Symmetry group||Dihedral (D7), order 2×7|
|Internal angle (degrees)||≈128.571°|
|Properties||convex, cyclic, equilateral, isogonal, isotoxal|
The heptagon is also occasionally referred to as the septagon, using "sept-" (an elision of septua-, a Latin-derived numerical prefix, rather than hepta-, a Greek-derived numerical prefix) together with the Greek suffix "-agon" meaning angle).
The area (A) of a regular heptagon of side length a is given by:
The exact form, starting from the zero of the polynomial x3 + x2 − 2x − 1, is given by:
where is the imaginary unit.
A regular heptagon is not constructible with compass and straightedge but is constructible with a marked ruler and compass. This type of construction is called a neusis construction. It is also constructible with compass, straightedge and angle trisector. The impossibility of straightedge and compass construction follows from the observation that is a zero of the irreducible cubic x3 + x2 − 2x − 1. Consequently this polynomial is the minimal polynomial of 2cos(2π⁄7), whereas the degree of the minimal polynomial for a constructible number must be a power of 2.
A neusis construction of the interior angle in a regular heptagon.
A neusis construction of the interior angle in a regular heptagon. (method by John Horton Conway)
An animation of an approximate compass-and-straightedge construction of a regular heptagon.
Another animation of an approximate construction.
Example to illustrating the error: At a circumscribed circle radius r = 10 km, the absolute error of the 1st side would be approximately -2,1 mm.
See also the calculation (Berechnung).
A decent approximation for practical use with an error of ≈0.2% is shown in the drawing. Let A lie on the circumference of the circumcircle. Draw arc BOC. Then gives an approximation for the edge of the heptagon.
A more accurate approximation
A regular heptagon with sides can be inscribed in a circle of the radius with an error of less than 0.00013%.
This follows from a rational approximation of .
Many police badges in the US have a 7:2 heptagram outline.
The United Kingdom currently (2011) has two heptagonal coins, the 50p and 20p pieces, and the Barbados Dollar is also heptagonal. The 20-eurocent coin has cavities placed similarly. Strictly, the shape of the coins is a Reuleaux heptagon, a curvilinear heptagon to make them curves of constant width: the sides are curved outwards so that the coin will roll smoothly in vending machines. Botswana pula coins in the denominations of 2 Pula, 1 Pula, 50 Thebe and 5 Thebe are also shaped as equilateral-curve heptagons. Coins in the shape of Reuleaux heptagons are in circulation in Mauritius, U.A.E., Tanzania, Samoa, Papua New Guinea, São Tomé and Príncipe, Haiti, Jamaica, Liberia, Ghana, the Gambia, Jordan, Jersey, Guernsey, Isle of Man, Gibraltar, Guyana, Solomon Islands, Falkland Islands and Saint Helena. The 1000 Kwacha coin of Zambia is a true heptagon.
The K7 complete graph is often drawn as a regular heptagon with all 21 edges connected. This graph also represents an orthographic projection of the 7 vertices and 21 edges of the 6-simplex. The regular skew polygon around the perimeter is called the petrie polygon.
Heptagon in natural structures
|Look up Heptagon in Wiktionary, the free dictionary.|
- Definition and properties of a heptagon With interactive animation
- Weisstein, Eric W., "Heptagon", MathWorld.
- Another approximate construction method
- Polygons – Heptagons
- Recently discovered and highly accurate approximation for the construction of a regular heptagon.