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The impossibility of straightedge and compass construction follows from the observation that is a zero of the irreducible cubic x 3 + x 2 − 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 pentagon is a five-sided polygon. A regular pentagon has 5 equal edges and 5 equal angles. In geometry, a polygon is traditionally a plane figure that is bounded by a finite chain of straight line segments closing in a loop to form a closed chain.
A regular digon has both angles equal and both sides equal and is represented by Schläfli symbol {2}. It may be constructed on a sphere as a pair of 180 degree arcs connecting antipodal points, when it forms a lune. The digon is the simplest abstract polytope of rank 2. A truncated digon, t{2} is a square, {4}. An alternated digon, h{2} is a ...
Four numbering schemes for the uniform polyhedra are in common use, distinguished by letters: [C] Coxeter et al., 1954, showed the convex forms as figures 15 through 32; three prismatic forms, figures 33–35; and the nonconvex forms, figures 36–92.
Chiliagon - 1,000 sides; Myriagon - 10,000 sides; Megagon - 1,000,000 sides; Star polygon – there are multiple types of stars Pentagram - star polygon with 5 sides; Hexagram – star polygon with 6 sides Star of David (example) Heptagram – star polygon with 7 sides; Octagram – star polygon with 8 sides Star of Lakshmi (example) Enneagram ...
Let the circle on AF as diameter cut OB in K, and let the circle whose centre is E and radius EK cut OA in N 3 and N 5; then if ordinates N 3 P 3, N 5 P 5 are drawn to the circle, the arcs AP 3, AP 5 will be 3/17 and 5/17 of the circumference." The point N 3 is very close to the center point of Thales' theorem over AF.
For the pentagon, this results in a polygon whose angles are all (360 − 108) / 2 = 126°. To find the number of sides this polygon has, the result is 360 / (180 − 126) = 6 2 ⁄ 3, which is not a whole number. Therefore, a pentagon cannot appear in any tiling made by regular polygons.
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]