# Basic Geometry

A **polygon ***is any closed figure with three or more straight sides*. "Closed" means that there are no gaping holes in it and that all sides connect together.

## These Are Polygons:

## These Are Not Polygons:

That's right, *a circle is not a polygon*. It doesn't have all the properties, and it's really hard to count the number of sides.

## Names of Some Common Polygons

Number of Sides | Number of Angles | Name |

3 | 3 | Triangle |

4 | 4 | Quadrilateral |

5 | 5 | Pentagon |

6 | 6 | Hexagon |

7 | 7 | Septagon (or Heptagon) |

8 | 8 | Octagon |

9 | 9 | Nonagon |

10 | 10 | Decagon |

12 | 12 | Dodecagon |

**Important Vocabulary**

Polygons have a lot of important vocabulary terms that come with them. You will need to memorize these, but don't worry, they aren't too difficult.

**Angle**: the shape formed when two rays meet at a common point. AKA "the corner."**Vertex**: the point where two rays meet; the corner point of a polygon. The plural of vertex is vertices.**Side**: the straight edge of a polygon.

**Equiangular**: a figure where all angles are equal in measure.**Equilateral**: a figure where all sides are equal in length.**Regular Polygon**: an equilateral, equiangular polygon.

## Regular Polygons

You're probably familiar with these shapes

## These Are Not Regular

## Base and Height of a Polygon

**Base**: the bottom side of a polygon.**Height**: the height of a polygon is the perpendicular distance from the top-most vertex to the base. The height and base ALWAYS form a right angle.