Calculate the circumference of a circleAs seen in the the number pi, the formula for the circumference of a circle is C = 2 × π × r or C = π × D If either r or D is known, you can calculate the circumference by simply substituting the known value for r in the formula. Use 3.14 for π. Examples #1 Calculate C if r = 2 inches C = 2 × π × r = 2 × 3.14 × 2 = 12.56 inches Examples #2 Calculate C if r = 4 inches C = 2 × π × r = 2 × 3.14 × 4 = 25.12 inches Examples #3 If D = 10 cm, calculate the circumference. You have two choices. You can first find r and then replace its value r is half the diameter, so r = 10 divided by 2 r = 5 cm C = 2 × π × r = 2 × 3.14 × 5 = 31.4 cm Otherwise, you can just use the formula C = π × D C = 3.14 × 10 = 31.4 cm Examples #4 The circumference of circle A is four times the circumference of circle B The diameter of circle B is 7. What is the diameter of circle A? Let C_{A} be the circumference of circle A Let C_{B} be the circumference of circle B Let D_{A} be the diameter of circle A Let D_{B} be the diameter of circle B Since the ratio of circumference to diameter is the same for all circles, you can use the following proportion to solve this problem
Things that we know: C_{A} = 4 × C_{B} D_{B} = 7 Replace these in the proportion
28 × C_{B} = D_{A} × C_{B} D_{A} = 28 since 28 × C_{B} = 28 × C_{B} no matter what C_{B} is Examples #5 If the circumference of a circle is 50.24 inches, calculate r. 




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