Area of a Circle

Find the circumference of each circle. Use 3.14 for .

1.

2.

Evaluate each expression.

3. 22 • 5

4. 32 – 12

5. 4(10)2

2 mi20 m

Find the radius

10 12 14

15 5

A = π r2r

Videos Jimmy Chang: http://www.youtube.com/watch?v=HSQh-HnQy6c&feature=player_embedded

[Simple – has 4 links to utube] Onlinemathlearning (only uses pi) : http://www.onlinemathlearning.com/circle-area.html

MathAtube (no calculator): http://167.128.19.241:9015/actionpage?

basictype=block&epochseconds=1267486017&requestedurl=http%3A%2F%2Fwww.bing.com%2Fvideos%2Fsearch%3Fq%3Darea%2Bof%2Ba%2Bcircle%2Bvideo%26FORM%3DVDRE&categorylist=164&categorydescriptionlist=Visual%20Search%20Engine&useripaddress=167.128.204.71&username=&actiontaken=block&actionreason=by-category&actionreasondata=164&reputationdesc=&replayhash=ZvnT3seAmzkOclczscmEMw%3D%3D

Advanced: http://www.mindbites.com/lesson/5318-math-expression-area-of-a-circle?FORM=VIRE3

Example

Find the areaA = π r2

A = (3.14)(5)2

A = 3.14 x 52

A = 3.14 x 25A = 78.5 cm2

5 cm

Find the area of the circle.Use 3.14 for π.

Find the length of the radius. 10 ÷ 2 = 5 Write the area formula. A = πr2

Substitute the known values. A ≈ (3.14)(5)2

Square 5. A ≈ (3.14)(25) Multiply. A ≈ 78.5

The area of A is approximately 78.5 ⊙ cm2.

Find the exact area of W.

Write the circle area formula. A = π r2

Substitute known values. A = π(3)2

Square 3. A = 9π

The exact area of W is 9π ft2.

1. Use 3.14 to find the area of each circle.

a. b.

2. What is the exact area of a circle with a radius of 5 inches?

4 cm

16 m

i

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 2r is the formula for circumference and r² is the formula for area?

Jaira is having pizza for her birthday party. She can make two small pizzas, each with a radius of 6 inches, or one large pizzawith 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) = 113.04 Large: A = (3.14)(12)2 = (3.14)(144) = 452.16

Multiply the area of a small pizza by 2. 113.04 • 2 = 226.08

One large pizza gives Jaira twice as much pizza as two small pizzas.