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## Geometry (all content)

### Course: Geometry (all content)>Unit 7

Lesson 8: Area and circumference of circles

# 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:
start text, C, i, r, c, u, m, f, e, r, e, n, c, e, end text, equals, pi, times, start text, d, i, a, m, e, t, e, r, end text
start text, C, i, r, c, u, m, f, e, r, e, n, c, e, end text, equals, 2, times, pi, times, start text, r, a, d, i, u, s, end text

### Example 1: Finding circumference when given diameter

Find the circumference of a circle with a diameter of start color #1fab54, 3, end color #1fab54.
The formula for the circumference of a circle that uses diameter is:
C, equals, pi, start color #1fab54, d, end color #1fab54
C, equals, pi, start color #1fab54, 3, end color #1fab54
C, equals, start color #11accd, 3, pi, end color #11accd
We can stop here and write our answer as start color #11accd, 3, pi, end color #11accd. Or we can plug in 3, point, 14 for pi and multiply.
C, equals, 3, point, 14, dot, 3
C, equals, start color #11accd, 9, point, 42, end color #11accd units
The circumference of the circle is start color #11accd, 3, pi, end color #11accd units or start color #11accd, 9, point, 42, end color #11accd units.

### Example 2: Finding circumference when given radius

Find the circumference of a circle with a radius of start color #7854ab, 8, end color #7854ab.
The formula for the circumference of a circle that uses radius is:
C, equals, 2, pi, start color #7854ab, r, end color #7854ab
C, equals, 2, pi, start color #7854ab, 8, end color #7854ab
C, equals, start color #11accd, 16, pi, end color #11accd
We can stop here and write our answer as start color #11accd, 16, pi, end color #11accd. Or we can plug in 3, point, 14 for pi and multiply.
C, equals, 3, point, 14, dot, 16
C, equals, start color #11accd, 50, point, 24, end color #11accd units
The circumference of the circle is start color #11accd, 16, pi, end color #11accd units or start color #11accd, 50, point, 24, end color #11accd units.

### Practice

Problem 1
• Current
Suppose the radius of a circle is start color #7854ab, 5, end color #7854ab. What is its circumference?
Either enter an exact answer in terms of pi or use 3, point, 14 for pi and enter your answer as a decimal.

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

## Want to join the conversation?

• It all depends on how you view easy as. If you haven't learnt circumference for a while or are new to the subject, some people can find it challenging.
• my district says this is 7th grade material
• To find area with circumference, you divide the circumference by pi, to get your diameter. Then, you find the radius by dividing the diameter by 2. You then multiply your radius by itself and multiply that product by pi.
Hope this Helps! :)
• why do we need to measure circles
• You don't really, but if you need to find your tire size, that's the way to do it. Also, a lot of math you won't really need for practicality, but you'll need it for high school which eventually leads up to college and your future job.
• Is the answer in the second practice question wrong?
My calculator said 9π is 28.27, not 28.26.

Ps: if it is wrong I don't see a way to report it.
• If you use π then you will get 28.27.
If you use 3.14 then you will get 28.26.
• What should I do ,I need help
• You should watch all of the videos again; they will help!

• hey i need help i don't understand can someone help me this is on my finals
• and for area mutiply the radius by itself then multiply it by pie (3.14)
• yall why are the questions from 4 years ago-
• The questions with the most votes are 4 years old, but if you go to the Sort by: and choose Recent, you will see questions that are 9, 10, and a month ago. Recent questions generally do not get many votes.