Circles &

Circumference

8th Grade Pre-AlgebraCircles & Circumference PPT

Molly Thompson

Parts of a Circle• Circle: the set of all points in a plane that are an equal

distance from a fixed point, called the center

• Circumference: the distance around the circle

radius

diameter

chord

Circumference, Diameter & π• Pi (π) is the ratio of the circumference of a circle divided

by its diameter. The value of pi is approximately 3.14.

C ≈ πd

• If the diameter of a circle is 10 in., then the circumference would be 31.4 in.

d= 10 in.

The Circumference Formula

• The formula for finding the circumference of a circle is… C ≈ πd. – To find the circumference, multiply 3.14 by the

value of the diameter

• C ≈ π d• C ≈ (3.14)(3 cm)• C ≈ 9.42 cm• The distance around the circle is 9.42 cm.

d = 3 cm

Circumference Examples

• Find the circumference of a circle with a diameter of 5.5 meters

C ≈ π dC ≈ (3.14)(5.5 m)C ≈ 17.27 m

Circumference Examples

• Find the circumference of a circle with a radius of 15 inches Since it takes two radii to make the diameter, 2(15) means the diameter is 30 inches.

C ≈ π dC ≈ (3.14)(30 in)C ≈ 94.2 inches

Real-life Circumference Example

Big Ben is a famous clock tower in London, England. The diameter of the clock face is 23 feet.

Find the circumference of Big Ben’s clock face.

C ≈π dC ≈ (3.14)(23 ft)C ≈ 72.22 feet