http://www.ck12.org Chapter 1. Basics of Geometry
Polygons
Polygon:Any closed planar figure that is made entirely of line segments that intersect at their endpoints.
Polygons can have any number of sides and angles, but the sides can never be curved. The segments are called the
sidesof the polygons, and the points where the segments intersect are calledvertices. The easiest way to identify a
polygon is to look for a closed figure with no curved sides.
Example 6:Which of the figures below is a polygon?
Solution:The easiest way to identify the polygon is to identify which shapes are not polygons.BandCeach have
at least one curved side, so they cannot be polygons.Dhas all straight sides, but one of the vertices is not at the
endpoint of the adjacent side, so it is not a polygon either.Ais the only polygon.
Example 7:Which of the figures below isnota polygon?
Solution:Cis a three-dimensional shape, so it does not lie within one plane, so it is not a polygon.
Convex and Concave Polygons
Polygons can be eitherconvexorconcave. Think of the term concave as referring to a cave, or “caving in”. A
concave polygon has a section that “points inward” toward the middle of the shape. All stars are concave polygons.
A convex polygon does not share this property.