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

$A\; =\; \backslash frac\{9\}\{4\}a^2\; \backslash cot\; \backslash frac\{\backslash pi\}\{9\}\; \backslash simeq\; 6.18182\; a^2.$

See also

• Enneagonal number
• Enneagram
• Geometry
• Trigonometry

External links

• Nonagon at MathWorld
• Enneagon at MathWorld

Polygons

Neuneck | dziewięciokąt | Eneágono | Yhdeksänkulmio | 九角形 | 正九邊形