Circumference & Areaof a Circle

Definitions

Circumference: The distance around a circle.

Diameter (d): a line that passes through the center of a circle that has its endpoints on the circle.

Radius (r): half the diameter

d = 20, then r = 10 r = 15, then d = 30

Circumference

To find the circumference multiply times the diameter, where equals 3.14

C = d

Diameter

Circumference

Example: Find the circumference

Diameter = 8 in

Circumference = d

C = 3.14(8)

C = 25.12

The circumference is 25.12 in

Circumference

Example: Find the circumference

radius = 10 cm

Circumference = d

Diameter = 2r = 2 (10) = 20

C = 3.14(20)

C = 62.8

The circumference is 62.8 cm

Your Turn…. Find the circumference

pg. 426 # 7-9 No naked numbers.7. C = d 8. C = d C = 3.14(12) C = 3.14(3) C = 37.7m C = 9.4 ft

9. C = d C = 3.14 (16) C = 50. 2 in

Area of Circles

The area of a circle is the product of and the square of the circle’s radius r.

A = r²

radius

Area of Circles

Example: Find the area of the circle

r = 3 m

A = r²A = 3.14(3)²A = 3.14(9)A = 28.26 m²The area of the circle is 28.26m²

Area of Circles

d = 8 in

A = r²Radius = 4 inA = 3.14(4)²A = 3.14(16)A = 50.24 in²

The area of the circle is 50.24in²

Lets Review

Perimeter = Sum of all sides

Circumference = d

Area of a Rectangle = ω

Area of a Triangle = bh

Area of a Circle = r²

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