ANSWERS: (1) C = π D

C = 3.14 x 10 inches

C = 31.4 inches

(2) C = π D

C = 3.14 x 7 inches

C = 21.98 inches

(3) D = _C_

π

D = 6.2832 feet

3.14

D = 2 feet

You should remember that the formula can be written these three ways:

π = _C_

or

C = π D

or

D = _C_

D

π

It depends on what you are trying to find.

DIAMETER = 2 x the radius

Look at the circle shown.

It should be easy to see that

a diameter is equal to 2

D = 2r or 1/2 D = r

You can substitute 2r for D in the formula C = π D. Then the formula for the

circumference of a circle is written:

C = 2 π r

or

C = 2 x 3.14 x r

(1) What is the diameter of a circle if the radius equals 6 inches?

D = 2r

(2) What is the circumference of the previous circle?

C = 2 π r