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Basic Geometry

Basic Geometry

Polygons

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:

Polygons

These Are Not Polygons:

Non-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 SidesNumber of AnglesName
33Triangle
44Quadrilateral
55Pentagon
66Hexagon
77Septagon (or Heptagon)
88Octagon
99Nonagon
1010Decagon
1212Dodecagon

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.

Angle Side Vertex Polygons

  • 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

Shapes

These Are Not Regular

Non-regular Polygons

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.

Height and Base Polygons

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