1. 2*(-9)=18 03.01.2019 Agenda Bell Ringer Bell ringer D.E.A.R.
Circles Video
Cornell notes: Area of a circle
Topic: Area and circumference of a Circle
E.Q. How do I calculate the Area of a circle?
Summarize Cornell Notes
Two truths, one lie Activity
2*(-9)=18
Format paper for Cornell notes
D.E.A.R.

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. Parts of a Circle
Radius-Straight line from the middle to the outside of a circle

4. Diameter-a line that cuts the circle in half from end to end

5. Circumference
Circumference is the distance around the circle. Like the perimeter.

6. Recap-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.

7. 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 .

8. 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.

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

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

11. 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

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

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

14. Example
3.14 · 36 = in²

15. Try

16. Try

17. Example
3.14 · 36 = in²

18. Try

19. Try

20. Example
Radius = 3

21. Try

24. Finding Radius and Diameter from Area
Just like you could find radius and diameter from circumference, you can find the radius and diameter from area by filling in known values and solving.

25. Example
Area = 𝛑r²
Area = cm²
Radius =
Diameter =

26. Area = cm²
Radius =
Diameter =

27. Area = cm²
Radius =
Diameter =

28. Reference