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In geometry, an enneagon or nonagon is a nine-sided polygon. The name enneagon comes from Greek enneagonon, (εννεα, nine + γωνον, corner), while nonagon comes from Latin (nonus, "ninth" + gonon). Enneagon is usage consistent with, for example, hexagon; in practice nonagon is often used.

A regular enneagon has internal angles of 140°. The area of a regular enneagon of side length a is given by

<math>A = \frac{9}{4}a^2 \cot \frac{\pi}{9} \simeq 6.18182 a^2.</math>

A regular enneagon is not constructible with compass and straightedge.

[edit] See also

• Enneagonal number
• Enneagram
• Geometry
• Trigonometry

[edit] External links

• Nonagon at MathWorld
• Enneagon at MathWorld

Polygons
Triangle | Quadrilateral | Pentagon | Hexagon | Heptagon | Octagon | Enneagon (Nonagon) | Decagon | Hendecagon | Dodecagon | Triskaidecagon | Pentadecagon | Hexadecagon | Heptadecagon | Enneadecagon | Icosagon | Icosihenagon | Tricontagon | Pentacontagon | Hectagon | Chiliagon | Myriagon

de:Neuneck

es:Eneágono ja:九角形 pl:Dziewięciokąt pt:Eneágono fi:Yhdeksänkulmio zh:正九邊形