Nonagon

The name enneagon comes from Greek enneagonon (εννεα, "nine" + γωνον (from γωνία = "corner")), and is arguably more correct,[1] though less common.

The area of a regular nonagon of side length a is given by where the radius r of the inscribed circle of the regular nonagon is and where R is the radius of its circumscribed circle: Although a regular nonagon is not constructible with compass and straightedge (as 9 = 32, which is not a product of distinct Fermat primes), there are very old methods of construction that produce very close approximations.

Cyclic symmetries in the middle column are labeled as g for their central gyration orders.

In the notation of symmetrohedron this tiling is called H(*;3;*;[2]) with H representing *632 hexagonal symmetry in the plane.

The K9 complete graph is often drawn as a regular enneagon with all 36 edges connected.

Nonagon, an animation from a neusis construction based on the angle trisection 120° by means of the Tomahawk , at the end 10 s break
Nonagon, a neusis construction based on a hexagon with trisection of the angle according to Archimedes [ 3 ]
Symmetries of a regular enneagon. Vertices are colored by their symmetry positions. Blue mirrors are drawn through vertices, and purple mirrors are drawn through edge. Gyration orders are given in the center.
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