Interior Angle | = | 60° |

Central Angle | = | 120° |

Slice Area | ≈ | 0.433 r^{2} |

Total Area | ≈ | 1.299 r^{2} |

Circumference | ≈ | 5.196 r |

A triangle is a polygon with 3 sides, and the sum of its interior angles is 180 degrees. A regular triangle is also called an equilateral triangle.

The values in the table above assume a regular polygon as in the picture. In a regular polygon, all angles and sides are equal, which means that a perfect circle can be drawn across all the vertices. The radius of this circle is called the circumradius, and is denoted **r** in the calculations above.

The more sides the regular polygon has, the more similar to a circle it becomes. As a result, the circumference gets closer and closer to 2 × π × r (≈ 6.283 r), and the area gets closer and closer to π × r^{2} (≈ 3.1415 r^{2}).

Here are all the polygons with twenty or less sides:

Number of Sides | Polygon Name |
---|---|

3 | Triangle |

4 | Quadrilateral |

5 | Pentagon |

6 | Hexagon |

7 | Heptagon |

8 | Octagon |

9 | Nonagon |

10 | Decagon |

11 | Hendecagon |

12 | Dodecagon |

13 | Tridecagon |

14 | Tetradecagon |

15 | Pentadecagon |

16 | Hexadecagon |

17 | Heptadecagon |

18 | Octadecagon |

19 | Enneadecagon |

20 | Icosagon |