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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). It is sometimes claimed that enneagon is the "proper" usage; however, in practice nonagon is most often used, just as Latin quadrilateral is preferred over Greek tetragon for four-sided polygons.
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>
See also
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