1. Name the polygons with the following number of sides:3 4 5 6 7 8 9 10 12
Triangle
Quadrilateral
Pentagon
Hexagon
Heptagon
Octagon
Nonagon
Decagon
Dodecagon

2. Geometry Mrs. King Unit 3, Lesson 4
Polygons
Geometry
Mrs. King
Unit 3, Lesson 4

3. Definition
Polygon: a closed plane figure with at least three sides.

4. Video: Polygons in Architecture

5. Name the polygon. Then identify its vertices, sides, and angles.
Its angles are named by the vertices, A, B, C, D, and E.
Its sides are AB, BC, CD, DE, and EA.
Its vertices are A, B, C, D, and E.
Possible names are ABCDE and EDCBA.

6. Without the use of your notes:
Matching names with number of sides:
Concentration game:

7. Definition
Convex: a polygon is convex if no diagonal contains points outside the polygon.

8. Definition
Concave: a polygon is concave if a diagonal contains points outside the polygon.

9. Theorem 2-3: Polygon Interior Angle-Sum Theorem
The sum of the measures of the interior angles of an n-gon is (n - 2)180.

10. Find the sum of the measures of the angles of a decagon.
A decagon has 10 sides, so n = 10.
Sum = (n – 2)(180)
= (10 – 2)(180)
= 8 • 180
= 1440

11. Practice
Find the sum of the measure of the interior angle of the following polygons:
15-gon
20-gon
Decagon
Dodecagon

12. (n-2) x 180 video
Cool Video!!

13. Find m  X in quadrilateral XYZW.
The figure has 4 sides, so n = 4.

14. Theorem 2-4: Polygon Exterior Angle-Sum Theorem
The sum of the measures of the exterior angles of a polygon, one at each vertex, is 360.

15. Video: General Definitions of Polygons

16. Definition Equilateral polygon: a polygon with all sides congruent.
Equiangular polygon: a polygon with all angles congruent.
Regular polygon: a polygon that is both equilateral and equiangular.