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EXAMPLE 1 Use the formula for circumference Find the indicated measures. Write circumference formula. Substitute 9 for r. Simplify. Use a calculator. = 2 π 9 = 18 56.55 a. Circumference of a circle with radius 9 centimeters SOLUTION The circumference is about 56.55 centimeters. ANSWER a.a. C = 2 πr

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EXAMPLE 1 Use the formula for circumference Write circumference formula. Substitute 26 for c. Divide each side by 2π. Use a calculator. r 4.14 = 2 πr26 = 2π2π r C = 2 πr The radius is about 4.14 meters. ANSWER Find the indicated measures. b. Radius of a circle with circumference 26 meters

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EXAMPLE 2 Use circumference to find distance traveled The dimensions of a car tire are shown at the right. To the nearest foot, how far does the tire travel when it makes 15 revolutions? STEP 1 Find the diameter of the tire STEP 2 Find the circumference of the tire Tire Revolutions SOLUTION d = 15 + 2 (5.5) = 26 in. C= πd = π(26)≈ 81.68 in.

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EXAMPLE 2 Use circumference to find distance traveled STEP 3 Find the distance the tire travels in 15 revolutions. In one revolution, the tire travels a distance equal to its circumference. In 15 revolutions, the tire travels a distance equal to 15 times its circumference. 15 81.68 in = 1225.2 in

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EXAMPLE 2 Use circumference to find distance traveled STEP 4 Use unit analysis. Change 1225.2 inches to feet. 1225.2 in. 1 ft 12 in. = 102.1 ft The tire travels approximately 102 feet. ANSWER

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GUIDED PRACTICE for Examples 1 and 2 1. Find the circumference of a circle with diameter 5 inches. Find the diameter of a circle with circumference 17 feet. Write circumference formula. Substitute 5 for d. Use a calculator. C = d = 5= 5 15.71 The circumference is about 15.71 in. SOLUTION

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GUIDED PRACTICE for Examples 1 and 2 Write circumference formula. C = d circumference = 17 feet Substitute 17 for c. Divide each side by π. Use a calculator. The Diameter is about 5.41 feet. r 5.41 = d 17 = d

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GUIDED PRACTICE for Examples 1 and 2 2. A car tire has a diameter of 28 inches. How many revolutions does the tire make while traveling 500 feet? SOLUTION STEP 1 Find the circumference of the tire C= πd = π(28)≈ 87.92 in. STEP 2 Use unit analysis. Change 500 feet to inches. 500 ft. 12 in. 1 ft = 6000 in.

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GUIDED PRACTICE for Examples 1 and 2 The tire makes 68.2 revolutions.ANSWER STEP 3 Find the number of revolutions N87.92 6000 = 68.24N

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