| 单词 | nonagon | ||||||||||||||||||||||||||||
| 释义 |
nonagon
|
||||||||||||||||||||||||||||
| 正九边形 | |
|---|---|
|
一个正九边形 | |
| 类型 | 正多边形 |
| 边 | 9 |
| 顶点 | 9 |
| 对角线 | 27 |
| 施莱夫利符号 | {9} |
| 考克斯特图 |
   |
| 对称群 | 二面体群 (D9), order 2×9 |
| 面积 |
9 4cotπ 9 ≈ 6.1818241937729a |
| 内角（度） | 140° |
| 内角和 | 1260° |
| 对偶 | 正九边形 (本身) |
| 特性 | 凸、圆内接多边形、等边多边形、等角多边形、isotoxal |
正九边形是九边、九角相等的多边形。内角为140°。若边长为
，面积
。若有正九边形
，
。它是继正七边形后另一个不能尺规作图的正多边形。
In geometry, a nonagon /ˈnɒnəɡɒn/ (or enneagon /ˈɛniːəɡɒn/) is a nine-sided polygon or 9-gon.
The name "nonagon" is a prefix hybrid formation, from Latin (nonus, "ninth" + gonon), used equivalently, attested already in the 16th century in French nonogone and in English from the 17th century. The name "enneagon" comes from Greek enneagonon (εννεα, "nine" + γωνον (from γωνία = "corner")), and is arguably more correct, though somewhat less common than "nonagon".
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