1. Geometry: Circles and Circumference
Lesson 6-9

2. Circles
A circle is the set of all points in a plane that are the same distance from a given point, called the center.

3. Circles The circumference (C) is the distance around a circle.
The diameter (d) is the distance across a circle through the center of the circle.
The radius (r) is the distance from the center to any point on the circle.

4. Circles
Pi is a non-terminating and non-repeating number represented by the Greek letter
 (pi)
3.14 is often used as an approximation for .

5. Circles Formulas for finding Circumference C = d C = 2r
If you are given the diameter in a problem use the formula with d. If you are given a problem with the radius use the formula with r.
Both formulas find the circumference.

6. Find Circumference Which formula for C will you use? C = 2r
C = inches
21 in

7. Your Turn Find the Circumference
Which formula for C will you use?
C = d
C = (4.5)
C = 14.1cm
4.5cm

8. Circles - Area Formula for finding the area of a circle: A = r
If you are given the diameter instead of the radius; divide the diameter by 2 to get the radius. 2

9. Find the Area D = 14 m r = ? 14 ÷ 2 = 7 m A =r A =(7 ) A = (49)
A = m
14m 2 2 2

10. Your Turn Find the Area A =r A =(5 ) A = (25) A = 78.5 ft 2 2 5 ft

11. Practice / Homework Practice: Lesson 6-9 pg 579 # 1-4
(Note the directions say to use 3.14 for pi. Do not use the pi button on the calculator. Type in the value 3.14)
Homework: Complete Lesson 6-9 #2-22 evens