Nonagon

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Regular nonagon (enneagon)
Regular polygon 9 annotated.svg
A regular nonagon (enneagon)
Type Regular polygon
Edges an vertices 9
Schläfli seembol {9}
Coxeter diagram CDel node 1.pngCDel 9.pngCDel node.png
Symmetry group Dihedral (D9), order 2×9
Internal angle (degrees) 140°
Dual polygon self
Properties convex, cyclic, equilateral, isogonal, isotoxal

In geometry, a nonagon /ˈnɒnəɡɒn/ (or enneagon /ˈɛnəɡɒn/) is a nine-sidit regular polygon.

The name "nonagon" is a prefix hybrid formation, frae Laitin (nonus, "ninth" + gonon), uised equivalently, attestit already in the 16t century in French nonogone an in Inglis frae the 17t century. The name "enneagon" comes frae Greek enneagonon (εννεα, "nine" + γωνον (from γωνία = "corner")), an is arguably mair correct, tho somewhit less common nor "nonagon".

A regular nonagon haes internal angles o 140°. The aurie o a regular nonagon o side length a is given bi

A = \frac{9}{4}a^2\cot\frac{\pi}{9}\simeq6.18182\,a^2.