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In geometry, a nonagon (/ ˈ n ɒ n ə ɡ ɒ n /) or enneagon (/ ˈ ɛ n i ə ɡ ɒ n /) is a nine-sided polygon or 9-gon. The name nonagon is a prefix hybrid formation , from Latin ( nonus , "ninth" + gonon ), used equivalently, attested already in the 16th century in French nonogone and in English from the 17th century.
The most common example is the pentagram, which has the same vertices as a pentagon, but connects alternating vertices. For an n-sided star polygon, the Schläfli symbol is modified to indicate the density or "starriness" m of the polygon, as {n/m}. If m is 2, for example, then every second point is
Close approximations to the regular hendecagon can be constructed. For instance, the ancient Greek mathematicians approximated the side length of a hendecagon inscribed in a unit circle as being 14/25 units long. [7] The hendecagon can be constructed exactly via neusis construction [8] and also via two-fold origami. [9]
In these polyhedra, the edges of one of the two side lengths of the kite meet at two "pole" vertices, while the edges of the other length form an equatorial zigzag path around the polyhedron. They are the dual polyhedra of the uniform antiprisms. [36] A commonly seen example is the pentagonal trapezohedron, used for ten-sided dice. [16]
One interior angle in a regular icositetragon is 165°, meaning that one exterior angle would be 15°.. The area of a regular icositetragon is: (with t = edge length) = = (+ + +).
An example of a convex polygon: a regular pentagon.. In geometry, a convex polygon is a polygon that is the boundary of a convex set.This means that the line segment between two points of the polygon is contained in the union of the interior and the boundary of the polygon.
An example of a concave polygon. A simple polygon that is not convex is called concave , [ 1 ] non-convex [ 2 ] or reentrant . [ 3 ] A concave polygon will always have at least one reflex interior angle —that is, an angle with a measure that is between 180 degrees and 360 degrees exclusive.
A convex equilateral pentagon can be described by two consecutive angles, which together determine the other angles. However, equilateral pentagons, and equilateral polygons with more than five sides, can also be concave, and if concave pentagons are allowed then two angles are no longer sufficient to determine the shape of the pentagon.