1. Circumference of Circles
Lesson 9.1.1
Ketchum

2. Lesson 9.1.1 What is circumference?
Circumference is a BIG word for the distance around a circle.
It is called “perimeter” for other polygons.

3. How do you calculate the circumference of a circle?
You first identify the measurement of the diameter.
The diameter is the distance from one side of the circle to the other, crossing through the center of the circle.
Once you know the diameter you multiply by pi.

4. What is Pi? What was the relationship?
Mathematicians found a relationship between the diameter of a circle and the circumference of the circle.
What was the relationship? …
A mathematical pattern we now call pi, other wise known as 3.14

5. C = d C = d C = 3.14(5) C = 15.7 cm C = 3.14(7) C = 21.98 in
Find the circumference:
Let’s Check this out!!!
Figure 1
C = d
C = 3.14(5)
5 cm
C = 15.7 cm
Try this one:
Figure 2
C = d
7 in
C = 3.14(7)
C = in

6. C = d C = d C= 3.14(30) C = 94.2 m C = 8.47 ft C = 3.14(2.7)
Your Turn!!!
Practice 1
Find the circumference:
C = d
30 m
C= 3.14(30)
C = 94.2 m
Practice 2
C = d
2.7 ft
C = 3.14(2.7)
C = 8.47 ft

7. What do we do if we aren’t given the diameter?
Sometimes you are given only half of the distance across a circle.
This is called the radius of a circle.

8. What to do with the radius?!
You multiply the radius by 2, (because 2 halves make a whole), and then multiply by pi, 3.14!
Radius →3(2) = 6 (diameter)
C = d
C = 3.14(6)
3 yd
C = yd

9. C = d C = d C = 3.14(10) C = 3.14(7) C = 21.98 in C = 31.4 km
Your Turn Now…
Blue radius
3.5(2) = 7 (Diameter)
Purple radius
5(2) = 10 (Diameter)
3.5 in
5 km
C = d
C = d
C = 3.14(10)
C = 3.14(7)
C = in
C = 31.4 km

10. Guided You Try C =  d C =  d C = 3.14(9) C = 3.14(16.4) C = 51.496 m

11. Guided You Try C =  d C =  d C = 3.14(9.5) C = 3.14(18) C = 56.52 m

12. You Try You Try C =  d C =  d C = 3.14(27.6) C = 3.14(50.2)
25.1 in
13.8 in
C =  d
C =  d
C = 3.14(27.6)
C = 3.14(50.2)
C = in
C = in
C = in
C = 86.7 in

13. You Try You Try C =  d C =  d C = 3.14(15) C = 3.14(4.4)
15 m
2.2 m
C =  d
C =  d
C = 3.14(15)
C = 3.14(4.4)
C = 47.1 m
C = m
C = 13.8 m

14. You Try You Try C =  d C =  d C = 3.14(56) C = 3.14(6.53)
6.53 yd
28 yd
C =  d
C =  d
C = 3.14(6.53)
C = 3.14(56)
C = yd
C = yd
C = yd
C = 20.5 yd