Circles Circumference and Area Algebra Mrs Ballard Finding

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. diameter s Where: d = diameter r = radius C = 2 r, diu or ra C = d

• Ex. 1 Find the circumference of the circle. 26 in. Let π = 3. 14; r = 26 Use C = 2πr C = 2(3. 14)(26) C = 163. 28 in

• Ex. 1 Find the exact circumference of the circle. 26 in. “exact” means do not change π to 3. 14 r = 26 Use C = 2πr C = 2(3. 14)(26) C = 163. 28 in. C = 2πr C = 2π(26) C = 52π in

Ex. 2 Find the circumference of the circle. Let π = 3. 14 6. 9 cm Find the exact circumference of the circle. C = πd C = 3. 14(6. 9) C = 21. 666 cm C = πd C = π(6. 9) C = 6. 9π cm

You try. A circle has a radius of 5 ft. Find the circumference. (Let π = 3. 14)

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. 14 d 3. 14 6 ft = d 3. 14

Area of a Circle A= 2 r Do not use diameter when finding the area of a circle. r

Ex. 4: Using the Area of a Circle • Find the exact area of P. 8 in. P r=8 A = r 2 = (8)(8) A = 64 64 ≈ 201. 06

Ex. 5: Find Radius Given Area • Find the radius of Z if A = 96 cm 2. (Let π = 3. 14) A = r 2 96 = (3. 14)r 2 3. 14 30. 56 = r 2 √ 30. 56 = √r 2 5. 53 r

Ex. 6 What is the area of a circular region whose diameter is 18 cm? A. 81π cm 2 C. 36π cm 2 d = 18 so r = 9 A = πr 2 A = π(9)(9) A = 81π cm 2 B. 36π cm D. 81π cm