Circumference

Warm UpFind the circumference of each circle. Use 3.14 or for π. Round to the nearest hundredth, if necessary.

For which problems did you use for π? Explain your choice.

Complete the table.Shape Perimeter

FormulaSide Length Perimeter

Square s = 7

Rectangle l = 5, w = 2

Equilateral Triangle

s = 2.5

Circle Circumference formula

d = 7r = 3.5

Circumference

• The diameter of a circle is twice the radius.

• The ratio of the circumference of a circle to the diameter is .

• Common approximations for π are 3.14 and .

• There are two equivalent formulas for the circumference of a circle: C = πd and C = 2πr, where d is the diameter of the circle and r is the radius of the circle.

Misty has a circular duck pond. If fencing costs $3.50 a foot, how much would it cost to put a fence around the circumference of

the pond? Find the answer using both 3.14 for π and for π. Then explain which

was easier to use in this situation.

A circular wading pool at a park has a radius of 21 feet. Find the

circumference of the pool.Use for π.

A circular concrete patio has a circumference of about 1,400 feet. A

gardener walks directly across the patio along a radius at a rate of 1.5 feet per

second. How long does it take him to get from the edge of the patio to the plant at

the center?

The Ferris wheel shown makes 12 revolutions per ride. How far

would someone travel during one ride?

A map of a public park shows a circular pond. There is a bridge

along a diameter of the pond that is 0.25 mi long. You walk across

the bridge, while your friend walks halfway around the pond to meet

you at the other side of the bridge. How much farther does your

friend walk?

Sam is placing rope lights around the edge of a circular patio with a

diameter of 18 feet. The lights come in lengths of 54 inches. How

many strands of lights does he need to surround the patio edge?

A circular path 2 feet wide has an inner diameter of 150 feet. How

much farther is it around the outer edge of the path than around the

inner edge?

Exit TicketFor 1–3, find the circumference of each circle. Use 3.14 or for π. Round to the nearest hundredth if necessary.1. a circle with radius 63 mm2. a circle with diameter 6.2 in.3. a circle with diameter 18 in.4. A circular satellite dish antenna has a circumference of about

62.8 inches. Find its diameter.5. The diameter of a bicycle’s tires is 2.5 feet. How many

revolutions of the tires does it take to travel 1 mile? Use 5,280 feet = 1 mile. Round to the nearest tenth.

6. The Jones family wants to enclose their circular garden, which has a radius of 62.5 ft., with a fence. To the nearest foot, what is the minimum length of fencing they need to buy? If the fence costs $2.50 per foot, what will the total cost be?