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### Course: 7th grade (Illustrative Mathematics)>Unit 3

Lesson 4: Lesson 5: Circumference and wheels

# 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.