Circumference of a Circle

Circumference - The distance around a circle

For every circle, the ratio of circumference C to diameter d is the same. This ratio, , is the Greek letter , called pi.

Pi is approximately equal to 3.14 or .

Cd

227

To find Circumference knowing diameter

To find Diameter use

C = d

To find Circumference knowing radius

C

C = 2r

Radius

Diameter

Circumference

Find the circumference of the circle to the nearest tenth. Use 3.14 for .

12 in.C = d

C 3.14 (12)

C 37.68

You know the diameter.

Substitute for and d.

Multiply.

EX1)

11 cmC = 2r

C 2 (3.14)(11)

C 69.08

EX2)

C = d

56.52 = 3.14 d

Substitute

Divide 3.14 on both sides of equation

EX3) The circumference of a circle is 56.52, what is the diameter.

3.14 3.14

18 = d

C = 2r

50.24 = 2(3.14)( r )

EX4) The circumference of a circle is 50.24, what is the radius.

3.14 3.14

16 = 2r

8 = r2 2

If the diameter or radius of a circle is a

multiple of 7, use for . 22 7

EX5)The diameter of a circular pond is 42 m. What is its circumference? Use 22

7for .

C = d You know the diameter.

C 227

· 42 Substitute 227

for and 42 for d.

C 227

· 421

C 227

· 421

6

1C 132

Write 42 as a fraction.

Simplify.

Multiply.

notes

Circumference of a Circle

Date _______________

Circumference -

For every circle, the ratio of circumference C to diameter d is the same. This ratio, , is the Greek letter , called pi.

Pi is approximately equal to ____ or .

Cd

To find Circumference knowing diameter

To find Diameter use

To find Circumference knowing radius

Find the circumference of the circle to the nearest tenth. Use 3.14 for .

12 in.

EX1)

11 cm

EX2)

EX3) The circumference of a circle is 56.52, what is the diameter.

EX4) The circumference of a circle is 50.24, what is the radius.

If the diameter or radius of a circle is a

multiple of 7, use for .

Helpful Hint

22 7

EX5)The diameter of a circular pond is 42 m. What is its circumference? Use 22

7for .