Enneagon
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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 straightedge and compass.