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Circles: Circumference and Area

Circles: Circumference and Area. Algebra Mrs. Ballard. Finding circumference. The circumference of a circle is the distance around the circle (perimeter). Circumference is represented by the letter C. C = d or C = 2r, Where: d = diameter r = radius. diameter. radius.

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Circles: Circumference and Area

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  1. Circles: Circumference and Area Algebra Mrs. Ballard

  2. Finding circumference • The circumference of a circle is the distance around the circle (perimeter).

  3. Circumference is represented by the letter C. C = d or C = 2r, Where: d = diameter r = radius diameter radius

  4. Let π = 3.14; r = 26 Use C = 2πr C = 2(3.14)(26) C = 163.28 in • Ex. 1 Find the circumference of the circle. 26 in. C = 2πr C = 2(3.14)(26) C = 163.28 in.

  5. “exact” means do not change π to 3.14 r = 26 Use C = 2πr C = 2π(26) C = 52π in • Ex. 1 Find the exact circumference of the circle. 26 in. C = 2πr C = 2(3.14)(26) C = 163.28 in.

  6. C = πd C = 3.14(6.9) C = 21.666 cm Ex. 2 Find the circumference of the circle. Let π = 3.14 Find the exact circumference of the circle. 6.9 cm C = πd C = π(6.9) C = 6.9π cm

  7. You try. A circle has a radius of 5 ft. Find the circumference. (Let π = 3.14)

  8. Sometimes you have to go “backwards.” A circle has a circumference of 18.84 ft. Find the diameter. (Let π = 3.14) C = πd 18.84= 3.14d 3.14 3.14 6ft = d

  9. Area of a Circle A = r2 Do not use diameter when finding the area of a circle. r

  10. Find the exact area of P. r = 8 A = r2 = (8)(8) A = 64 Ex. 4: Using the Area of a Circle 8 in. P 64 ≈ 201.06

  11. Find the radius of Z if A = 96 cm2. (Let π = 3.14) A = r2 96 = (3.14)r2 3.14 3.14 30.56 = r2 √30.56 =√r2 5.53  r Ex. 5: Find Radius Given Area

  12. Ex. 6 What is the area of a circular region whose diameter is 18 cm? • 81π cm2 B. 36π cm • 36π cm2 D. 81π cm d = 18 so r = 9 A = πr2 A = π(9)(9) A = 81π cm2

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