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Circles & Circumference 8 th Grade Pre-Algebra Circles & Circumference PPT Molly Thompson

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Parts of a Circle Circle: the set of all points in a plane that are an equal distance from a fixed point, called the center Circumference: the distance around the circle radius diameterdiameter chord

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Circumference, Diameter & π Pi (π) is the ratio of the circumference of a circle divided by its diameter. The value of pi is approximately 3.14. C ≈ πd If the diameter of a circle is 10 in., then the circumference would be 31.4 in. d= 10 in.

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The Circumference Formula The formula for finding the circumference of a circle is… C ≈ πd. – To find the circumference, multiply 3.14 by the value of the diameter C ≈ π d C ≈ (3.14)(3 cm) C ≈ 9.42 cm The distance around the circle is 9.42 cm. d = 3 cm

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Circumference Examples Find the circumference of a circle with a diameter of 5.5 meters C ≈ π d C ≈ (3.14)(5.5 m) C ≈ 17.27 m

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Circumference Examples Find the circumference of a circle with a radius of 15 inches Since it takes two radii to make the diameter, 2(15) means the diameter is 30 inches. C ≈ π d C ≈ (3.14)(30 in) C ≈ 94.2 inches

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Real-life Circumference Example Big Ben is a famous clock tower in London, England. The diameter of the clock face is 23 feet. Find the circumference of Big Ben’s clock face. C ≈π d C ≈ (3.14)(23 ft) C ≈ 72.22 feet

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