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In geometry, an icosihenagon or 21-gon is a twenty-one-sided polygon. The sum of any icosihenagon's interior angles is 3420 degrees. An icosihenagon has 189 diagonals.
| Regular icosihenagon | |
|---|---|
 A regular icosihenagon | |
| Type | Regular polygon |
| Edges and vertices | 21 |
| Schläfli symbol | {21} |
| Coxeter–Dynkin diagrams |     |
| Symmetry group | Dihedral (D21), order 2×21 |
| Internal angle (degrees) | ≈162.857° |
| Properties | Convex, cyclic, equilateral, isogonal, isotoxal |
| Dual polygon | Self |
Regular form
An angle of a regular icosihenagon is degrees.
Since (21) = 12 is 3-smooth but not power of 2, thus the regular icosihenagon is constructible using neusis, or an angle trisector, but not constructible using a compass and straightedge.
See also
- Heptagon
- Tetradecagon (14-sided)
- Icosioctagon (28-sided)
- Tetracontadigon (42-sided)
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