Calculate the circumference of a circle
As seen in the the number pi, the formula for the circumference of a circle is
C = 2 × π × r or C = π × D
If either r or D is known, you can calculate the circumference by simply substituting the known value for r in the formula. Use 3.14 for π.
Calculate C if r = 2 inches
C = 2 × π × r = 2 × 3.14 × 2 = 12.56 inches
Calculate C if r = 4 inches
C = 2 × π × r = 2 × 3.14 × 4 = 25.12 inches
If D = 10 cm, calculate the circumference.
You have two choices. You can first find r and then replace its value
r is half the diameter, so r = 10 divided by 2
r = 5 cm
C = 2 × π × r = 2 × 3.14 × 5 = 31.4 cm
Otherwise, you can just use the formula C = π × D
C = 3.14 × 10 = 31.4 cm
The circumference of circle A is four times the circumference of circle B
The diameter of circle B is 7. What is the diameter of circle A?
Let CA be the circumference of circle A
Let CB be the circumference of circle B
Let DA be the diameter of circle A
Let DB be the diameter of circle B
Since the ratio of circumference to diameter is the same for all circles, you can use the following proportion to solve this problem
Things that we know:
CA = 4 × CB
DB = 7
Replace these in the proportion
28 × CB = DA × CB
DA = 28 since 28 × CB = 28 × CB no matter what CB is
If the circumference of a circle is 50.24 inches, calculate r.
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