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Circumference review

Review the basics of circumference and try some practice problems.

What is circumference?

Circumference is the distance around the outside of a circle (its perimeter!).
Want a review of circle vocabulary terms (like pi, radius, and diameter)? Check out this video.

Finding circumference of a circle

To find the circumference of a circle, we can use either of the following formulas:
Circumference=π×diameter
Circumference=2×π×radius
Want to know why these formulas work? Check out this article.

Example 1: Finding circumference when given diameter

Find the circumference of a circle with a diameter of 3.
The formula for the circumference of a circle that uses diameter is:
C=πd
C=π3
C=3π
We can stop here and write our answer as 3π. Or we can plug in 3.14 for π and multiply.
C=3.143
C=9.42 units
The circumference of the circle is 3π units or 9.42 units.

Example 2: Finding circumference when given radius

Find the circumference of a circle with a radius of 8.
The formula for the circumference of a circle that uses radius is:
C=2πr
C=2π8
C=16π
We can stop here and write our answer as 16π. Or we can plug in 3.14 for π and multiply.
C=3.1416
C=50.24 units
The circumference of the circle is 16π units or 50.24 units.

Practice

Problem 1
Suppose the radius of a circle is 5. What is its circumference?
Either enter an exact answer in terms of π or use 3.14 for π and enter your answer as a decimal.

Want to try some more circumference of circle problems? Check out this exercise.

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