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In mathematics, a constructible polygon is a regular polygon that can be constructed with compass and straightedge. For example, a regular pentagon is constructible with compass and straightedge while a regular heptagon is not. There are infinitely many constructible polygons, but only 31 with an odd number of sides are known.
The following method of construction uses Carlyle circles, as shown below. Based on the construction of the regular 17-gon, one can readily construct n -gons with n being the product of 17 with 3 or 5 (or both) and any power of 2: a regular 51-gon, 85-gon or 255-gon and any regular n -gon with 2 h times as many sides.
This is impossible in the general case. For example, the angle 2 π /5 radians (72° = 360°/5) can be trisected, but the angle of π /3 radians (60°) cannot be trisected. [8] The general trisection problem is also easily solved when a straightedge with two marks on it is allowed (a neusis construction).
Some regular polygons are easy to construct with compass and straightedge; other regular polygons are not constructible at all. The ancient Greek mathematicians knew how to construct a regular polygon with 3, 4, or 5 sides, [11]: p. xi and they knew how to construct a regular polygon with double the number of sides of a given regular polygon.
With a construction system that can trisect angles, such as mathematical origami, more numbers of sides are possible, using Pierpont primes in place of Fermat primes, including -gons for equal to 7, 13, 14, 17, 19, etc. [6] Geometric Origami provides explicit folding instructions for 15 different regular polygons, including those with 3, 5, 6 ...
Any straight-sided digon is regular even though it is degenerate, because its two edges are the same length and its two angles are equal (both being zero degrees). As such, the regular digon is a constructible polygon. [3] Some definitions of a polygon do not consider the digon to be a proper polygon because of its degeneracy in the Euclidean ...
In geometry, a heptagon or septagon is a seven-sided polygon or 7-gon.. The heptagon is sometimes referred to as the septagon, using "sept-" (an elision of septua-, a Latin-derived numerical prefix, rather than hepta-, a Greek-derived numerical prefix; both are cognate) together with the Greek suffix "-agon" meaning angle.
The following construction description is given by T. Drummond from 1800: [10] Draw the radius A B , bisect it in C —with an opening of the compasses equal to half the radius, upon A and C as centres describe the arcs C D I and A D —with the distance I D upon I describe the arc D O and draw the line C O , which will be the extent of one ...