1. Area of a Circle

2. Find the circumference of each circle. Use 3.14 for .
Evaluate each expression.
22 • 5
32 – 12
4(10)2
2 mi
20 m

3. Find the radius 10 12 14 5 15

4. Area of a Circle
A = π r2 r

5. Videos
Jimmy Chang:
[Simple – has 4 links to utube] Onlinemathlearning (only uses pi) :
MathAtube (no calculator): 3A%2F%2Fwww.bing.com%2Fvideos%2Fsearch%3Fq%3Darea%2Bof%2Ba%2Bcircle%2Bvideo%26FO RM%3DVDRE&categorylist=164&categorydescriptionlist=Visual%20Search%20Engine&useripaddress= &username=&actiontaken=block&actionreason=by- category&actionreasondata=164&reputationdesc=&replayhash=ZvnT3seAmzkOclczscmEMw%3D%3D
Advanced:

6. Find the area A = π r2 A = (3.14)(5)2 A = 3.14 x 52 A = 3.14 x 25
Example
5 cm
Find the area
A = π r2
A = (3.14)(5)2
A = 3.14 x 52
A = 3.14 x 25
A = 78.5 cm2

7. Example 1 Find the area of the circle. Use 3.14 for π.
Find the length of the radius ÷ 2 = 5
Write the area formula. A = πr2
Substitute the known values. A ≈ (3.14)(5)2
Square A ≈ (3.14)(25)
Multiply. A ≈ 78.5
The area of ⊙A is approximately 78.5 cm2.

8. Example 3 Find the exact area of  W.
Write the circle area formula. A = π r2
Substitute known values. A = π(3)2
Square A = 9π
The exact area of  W is 9π ft2.

9. Use 3.14 to find the area of each circle. a. b.
What is the exact area of a circle with a radius of 5 inches?
4 cm
16 mi

10. Communication Prompt
What is the same about the formulas for area and circumference of a circle? What is different about the formulas for area and circumference of a circle?
How will you remember that 2r is the formula for circumference and r² is the formula for area?

11. Example 2 Write the circle area formula. A = πr2
Jaira is having pizza for her birthday party. She can make two
small pizzas, each with a radius of 6 inches, or one large pizza
with a radius of 12 inches. Which option will give her more
square inches of pizza?
Write the circle area formula. A = πr2
Substitute known values and find the area of each size.
Small: A = (3.14)(6)2 = (3.14)(36) =
Large: A = (3.14)(12)2 = (3.14)(144) =
Multiply the area of a small pizza by • 2 =
One large pizza gives Jaira twice as much pizza as two small pizzas.