1. Warm-Up What is the perimeter of a regular decagon with side length s = 6?

2. Geometry 9-2 Developing Formulas for Circles and Regular Polygons

3. Definitions A circle is the collection of points in a plane that are a fixed distance from a point called the center of the circle. A circle is named by the symbol  and its center.  A has radius r = AB and diameter d = CD.

4. Circumference and Area

5. Example Find the area and circumference of  K.

6. Example What is the area of the shaded region? 4 in.

7. Definition The center of a regular polygon is equidistant from the vertices. The apothem is the distance from the center to a side. The apothem of the figure is BC.

8. Formula

9. Examples What is the area of a regular triangle with side length s = 4m and apothem a = 1.5m. What is the area of a regular octagon with side length s = 5 and apothem a = 2.