1. Polygons
Introduction
Irma Crespo 2010

2. What is a polygon?
A polygon is a simple closed figure in a plane formed by three or more line segments.

3. A Polygon
closed figure
bounded by straight line segments
YES!!!

4. Which are the polygons? Polygons Non-polygons
close figures; formed by straight line segments
open figures; formed by curves or lines

5. Polygons are grouped by the number of sides.
Name of Polygon
Number of Sides
Triangle 3
Quadrilateral 4
Pentagon 5
Hexagon 6
Heptagon 7
Octagon 8
Nonagon 9
Decagon 10
Undecagon 11
Dodecagon 12
n-gon n
What is the fewest number of sides a polygon can have? 3
What is the largest number of sides a polygon can have? n

6. Polygons are either convex or concave.
Concave: some line segments connecting any two points on the shape pass outside the shape
Convex: line segments connecting any two points on the shape lie entirely inside the shape

7. Quickies
Parallel lines are lines that are across each other but never meet.
90° angles: L-shape, no matter how you turn it.

8. Quadrilateral Family Tree
(4 sides)
Trapezoid
(one pair of parallel sides)
Parallelogram
(two pairs of parallel sides)
Rhombus
(all side lengths are equal)
Rectangle
(four 90° angles)
Square
(all side lengths are equal and four 90° angles)

9. Summary
A polygon is a simple closed figure in a plane formed by three or more line segments.
Polygons are grouped based on the number of sides.
Polygons are either convex or concave.
Quadrilaterals are polygons of four sides.

10. Exit Slip
Complete the family tree on the Activity Sheet, “Roots of a Square.”
Don’t forget to write your name, date, and hour.
This is an extra credit point towards homework or quiz.
Turn it in before leaving class.

11. Practice Worksheet Complete the practice worksheet.
Work with a partner or on your own.
Submit completed worksheet for grading.
Solutions are discussed the next day.

12. Main Resources http://www.absorblearning.com/mathematics/demo/units
Larson, Boswell, and Stiff. Geometry Illinois: McDougall Littell.
PowerPoint created by Irma Crespo.
University of Michigan-Dearborn, School of Education. Winter 2010.