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In geometry, a decagon (from the Greek δέκα déka and γωνία gonía, "ten angles") is a ten-sided polygon or 10-gon. [1] The total sum of the interior angles of a simple decagon is 1440°. Regular decagon
A regular triangle, decagon, and pentadecagon can completely fill a plane vertex. However, due to the triangle's odd number of sides, the figures cannot alternate around the triangle, so the vertex cannot produce a semiregular tiling.
Symmetries of a regular tridecagon. Vertices are colored by their symmetry positions. Blue mirrors are drawn through vertices and edge. Gyration orders are given in the center. The regular tridecagon has Dih 13 symmetry, order 26. Since 13 is a prime number there is one subgroup with dihedral symmetry: Dih 1, and 2 cyclic group symmetries: Z 13 ...
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: =
The regular hexadecagon has Dih 16 symmetry, order 32. There are 4 dihedral subgroups: Dih 8, Dih 4, Dih 2, and Dih 1, and 5 cyclic subgroups: Z 16, Z 8, Z 4, Z 2, and Z 1, the last implying no symmetry.
Apothem of a hexagon Graphs of side, s; apothem, a; and area, A of regular polygons of n sides and circumradius 1, with the base, b of a rectangle with the same area. The green line shows the case n = 6. The apothem (sometimes abbreviated as apo [1]) of a regular polygon is a line segment from the center to the midpoint of one of its sides.
As 14 = 2 × 7, a regular tetradecagon cannot be constructed using a compass and straightedge. [1] However, it is constructible using neusis with use of the angle trisector, [2] or with a marked ruler, [3] as shown in the following two examples.
A full proof of necessity was given by Pierre Wantzel in 1837. The result is known as the Gauss–Wantzel theorem . Equivalently, a regular n -gon is constructible if and only if the cosine of its common angle is a constructible number —that is, can be written in terms of the four basic arithmetic operations and the extraction of square roots.