## Polygons

A polygon is a plane figure made up of three or more line segments which intersect only at their endpoints and in which each endpoint is on exactly two line segments.

The line segments are called the sides of the polygon. Each endpoint is called a vertex of the polygon.

## Convex Polygon

A polygon is convex if for any two points, A and B, in the interior of the polygon, the line segment AB is also in the interior of the polygon.

## Concave Polygon

A polygon which is not convex is called concave.

Can you find two points in the interior of this polygon such that the line segment joining them is not entirely in the interior of the polygon?

## Regular Polygon

A regular polygon is a polygon in which all the sides are equal and all the angles are equal. For example, the equilateral triangle is a regular polygon.

The angles are measured on the interior of the polygon. Is it possible to draw a regular polygon that is concave?

More on Regular Polygons

## More terms

#### Plane Figure

A plane figure is a figure in a plane, that is, on a flat surface.

#### Interior and Exterior

A closed plane figure divides the plane into two regions, the interior and the exterior. The interior is what we commonly think of as the inside. The exterior is what we commonly think of as the outside.

A more precise definition depends on whether or not a line can be drawn in the region.

Can a line be drawn in the interior of a closed plane figure?

Can a line be drawn in the exterior of a closed plane figure?