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
22
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