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Circle Circumference and Radius

Learn how to calculate the circumference and radius of a circle, and practice solving problems related to circumference and area.

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Circle Circumference and Radius

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  1. Do Now: • Solve 5 + 2[(92 – 4) + (23 + 1)] • Solve and graph x + 11>25 • Find the LCM of 48 and 90.

  2. CIRCLE circumference C radius r diameter d Center - the point located in the middle of a circle Radius - the distance from the center to any point on the circle Diameter - the distance across the circle through the center. Circumference: the distance around the circle

  3. Radius r = 3 cm Diameter d = 6 cm Diameter is twice the length of the radius. d r = 2

  4. Find the diameter or radius of each object. 3. 1. 2. 5 in 42 cm 8mm Radius = 4 mm 21 cm Radius = Diameter = 10 in Find the Diameter or Radius of a Circle

  5. Circumference of a Circle C = dπ C = 2rπ Find the circumference of each object. Round your answer to the nearest tenth. 5 in 42 cm 3. 2. 1. 8mm C = dπ C = 2rπ C = dπ = 42π = 2(5)π = 8π ≈ 131.9cm ≈ 25.1mm = 10π ≈ 31.4in Find the Circumference of a Circle

  6. Circumference of a Circle C = dπ C = 2rπ Find the circumference of each object. Round your answer to the nearest tenth. C = dπ 1. = 55π d = 55 cm ≈ 172.8cm C = 2r π 2. = 2(18) π r = 18 in = 36π ≈ 113.1in 3. r = 2.5 ft C = 2r π = 2(2.5) π = 5π ≈ 15.7ft Find the Circumference of a Circle

  7. Practice - Find the circumference of each circle. Round your answer to the nearest tenth. 1. 2. 3. 19 in 5 ft 2 mm 4. 5. 6. 100 yd 26 cm 1km Find the Circumference of a Circle

  8. Area of a Circle A = πr2 Find the area if each object. Round your answer to the nearest hundredth. 5 in 42 cm 3. 1. 2. 8mm A = πr2 A = πr2 A = πr2 A = π42 A = π52 A = π212 A = 16π A = 25π A = 441π A ≈ 50.27mm2 A ≈ 78.54in2 A ≈ 1,385.44cm2 Find the Area of a Circle

  9. Practice - Find the area. Round your answers to the nearest hundredth. 1. 2. 3. 10cm 4 mm 6.3 ft A = πr2 A = πr2 A = πr2 A = π52 A = π22 A = π6.32 A = 25π A = 4π A = 39.69π A ≈ 78.54cm2 A ≈ 12.57mm2 A ≈ 124.69ft2 Find the Area of a Circle

  10. Practice - Find the circumference and area. Leave your answer in terms of π. 3. r = 5 in 1. d = 6 cm 2. d = 18 mm C = dπ C = dπ C = dπ C = 6π cm C = 18π mm C = 10π in A = πr2 A = πr2 A = πr2 A = π32 A = π92 A = π52 A = 9π cm2 A = 81π mm2 A = 25π in2 Find the Area and Circumference of a Circle

  11. Find the radius of the circle when given the circumference. C = 2∏r 1. C = 16 ∏ 2. C = 7 ∏ 3. C = 50 ∏ 2∏r = 16 ∏ 2∏r = 7∏ 2∏r = 50∏ 2 2 2 2 2 2 ∏r = 8 ∏ ∏r = 3.5 ∏ ∏r = 25 ∏ ∏ ∏ ∏ ∏ ∏ ∏ r = 8 r = 3.5 r = 25 Find the Radius of the Circle given the Circumference

  12. Find the radius of the circle when given the circumference. C = 2∏r 1. C = 10 ∏ 2. C = 44 ∏ 3. C = 200 ∏ 2∏r = 44∏ 2∏r = 200∏ 2∏r = 10∏ 2 2 2 2 2 2 ∏r = 22 ∏ ∏r = 100 ∏ ∏r = 5 ∏ ∏ ∏ ∏ ∏ ∏ ∏ r = 22 r = 100 r = 5 Find the Radius of the Circle given the Circumference

  13. Jordan sews a lace border 42π inches long around the edge of a circular tablecloth? What is the length of the radius from the inside edge of the lace to the center of the circular tablecloth? C = 2∏r 42π C = 42∏ 2∏r = 42∏ 2 2 ∏r = 21 ∏ ∏ ∏ The length of the radius is 21 inches long. r = 21 Find the Radius of the Circle given the Circumference

  14. A circular rug has a circumference of 6π feet. What is the radius of the rug? C = 2∏r C = 6∏ 2∏r = 6∏ 2 2 ∏r = 3 ∏ ∏ ∏ The radius of the rug is 3 feet. r = 3 Find the Radius of the Circle given the Circumference

  15. The circumference of the circle below is 25.12 centimeters. (2006) What is the best estimate for the length of the radius of the circle? a) 3 centimeters b) 4 centimeters c) 8 centimeters d)16 centimeters C = 2∏r radius C = 25.12 2∏r = 25.12 2 2 ∏r = 12.56 3.14r = 12.56 3.14 3.14 r = 4

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