Area of a Circle.

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Presentation transcript:

Area of a Circle

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

Find the radius 10 12 14 5 15

Area of a Circle A = π r2 r

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%26FO RM%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

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

Example 1 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.

Example 3 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.

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

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?

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) = 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.