2. Polygon Closed plane figure with at least three sides The sides intersect only at their endpoints No adjacent sides are collinear To name a polygon –Start at any vertex and list vertices consecutively.

3. Names of Polygons Number of SidesName of Polygon 3Triangle 4Quadrilateral 5Pentagon 6Hexagon 7Heptagon

4. Names of Polygons (con’t) Number of SidesName of Polygon 8Octagon 9Nonagon 10Decagon 12Dodecagon 20Icosagon

5. PolygonNumber of sides (n) Number of triangles Sum of interior angle measures Quadrilateral422(180)=360 Pentagon Hexagon Heptagon Octagon Nonagon

6. Theorem 2-3 Polygon Interior Angle-Sum Theorem The sum of the measures of the interior angles of any polygon with n sides (n – 2) 180

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

8. Regular Polygon A polygon with all sides congruent and all angles congruent. A polygon that is both equilateral and equiangular

9. To find each interior angle measure of regular polygon The measure of each interior angle of a regular polygon with n sides (n – 2) 180 n

10. Examples