EXAMPLE 1 Using Radius to Find Circumference

Find the circumference of a circle with a radius of 11 meters.

SOLUTION

C = 2πr Circumference formula

2(3.14)(11) Substitute 3.14 for π and 11 for r.

= 69.08 Multiply.

ANSWER

The circumference is about 69.08 meters.

EXAMPLE 2 Using Diameter to Find Circumference

SOLUTION

The measure shown is a diameter, so use the circumference formula that involves the diameter.

C = πd Circumference formula

Substitute. Use for π because 21 is divisible by 7.

22 7

22

71

. 213

Multiply= 66

Find the circumference of the circle.

ANSWER The circumference is about 66 feet.

EXAMPLE 3 Standardized Test Practice

SOLUTION

You are asked to find the diameter, so use the circumference formula that involves diameter.

C = πd Circumference formula

423.9 3.14d Substitute 423.9 for C and 3.14 for π.

423.9 3.14

=3.14d 3.14

Divide each side by 3.14.

135 = d Simplify.

ANSWERThe diameter is about 135 feet. The correct answer is C.

GUIDED PRACTICE for Examples 1, 2, and 3

Find the circumference of the circle. Use 3.14 or for π . Explain your choice of π .

22 7

1.

Use for π because 28 is divisible by 7.

22 7

ANSWER The circumference is about 176 mi.

GUIDED PRACTICE for Examples 1, 2, and 3

2.

Use 3.14 for π because 9.5 is not divisible by 7.

ANSWER The circumference is about 59.66 m.

Find the circumference of the circle. Use 3.14 or for π . Explain your choice of π .

22 7

GUIDED PRACTICE for Examples 1, 2, and 3

3.

Use 3.14 for π because 16 is not divisible by 7.

ANSWER The circumference is about 50.24 cm.

Find the circumference of the circle. Use 3.14 or for π . Explain your choice of π .

22 7

GUIDED PRACTICE for Examples 1, 2, and 3

4. diameter = 32 in.

Use 3.14 for π because 32 is not divisible by 7.

ANSWER The circumference is about 100.48 cm.

GUIDED PRACTICE for Examples 1, 2, and 3

5. diameter = 140 ft.

ANSWER The circumference is about 440 ft.

Use for π because 140 is divisible by 7.

227

GUIDED PRACTICE for Examples 1, 2, and 3

Use 3.14 for π because 1.5 is not divisible by 7.

ANSWER The circumference is about 9.42 cm.

6. radius = 1.5 km

GUIDED PRACTICE for Examples 1, 2, and 3

7. What If? In Example 3, suppose the circumference of the Ferris wheel is 462 feet. What is the diameter of the Ferris wheel?

ANSWER The diameter is about 147 feet.