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Circles: Circumference and Area

Learn how to find the circumference and area of circles using algebraic formulas and Mrs. Ballard's guidance. Practice solving examples involving radius, diameter, and π. Get ready to master circle calculations!

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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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