LESSON 3: Polygons

- most of the basic shapes, such as triangle, square, retanglr, are parts of a larger subset of closed figures bounded by broken lines called polygons. The term "polygon" is a combination of two Greek words, "poly" which means "many" and "gonia" which means "angle".

- polygon is a two dimensional closed figure bounded by straight line segments.

Parts of a Polygon

1. Side or edge is one of the line segments that make up the polygon.

2. Vertex is a point where the sides meet.

3. Diagonal is a line connecting two non-adjacent vertices.

4. Interior angle is the angle formed by two adjacent sides inside the polygon.

5. Exterior angle is the angle formed by two adjacent sides outside the polygon.

6. Apothem (of a regular polygon) is the segment connecting the center of a polygon and the midpoint of a side. The apothem is a perpendicular bisector of the opposite side.

7. Central angle (of a regular polygon) is the angle subtended by a side about the center.

Names of Polygons

		If it is a Regular Polygon...
Name	Sides	Shape	Interior Angle
Triangle (or Trigon)	3		60°
Quadrilateral (or Tetragon)	4		90°
Pentagon	5		108°
Hexagon	6		120°
Heptagon (or Septagon)	7		128.571°
Octagon	8		135°
Nonagon (or Enneagon)	9		140°
Decagon	10		144°
Hendecagon (or Undecagon)	11		147.273°
Dodecagon	12		150°
Triskaidecagon	13		152.308°
Tetrakaidecagon	14		154.286°
Pentadecagon	15		156°
Hexakaidecagon	16		157.5°
Heptadecagon	17		158.824°
Octakaidecagon	18		160°
Enneadecagon	19		161.053°
Icosagon	20		162°
Triacontagon	30		168°
Tetracontagon	40		171°
Pentacontagon	50		172.8°
Hexacontagon	60		174°
Heptacontagon	70		174.857°
Octacontagon	80		175.5°
Enneacontagon	90		176°
Hectagon	100		176.4°
Chiliagon	1,000		179.64°
Myriagon	10,000		179.964°
Megagon	1,000,000		~180°
Googolgon	10100		~180°
n-gon	n		(n-2) × 180° / n