# LESSON: Circumference of Circles

## READ: Find the Diameter or Radius of a Circle Given Circumference

**Find the Diameter or Radius of a Circle Given Circumference**

**What happens if you are given the circumference but not the radius or the diameter? Can you still solve for one or the other?**

**Working in this way is a bit tricky and will require us to work as detectives once again. You will have to work backwards to figure out the radius and/or the diameter when given only the circumference to work with.**

Example 1

*Find the diameter of a circle with a circumference of 21.98 feet.*

**To work on this problem, we will need our formula for finding the circumference of a circle.**

**Next, we fill in the given information.**

**To solve this problem we need to figure out what times 3.14 will give us 21.98. To do this, we divide 21.98 by 3.14.**

Remember dividing decimals? First, we move the decimal point two places to make our divisor a whole number. Then we can divide as usual.

**The diameter of this circle is 7 feet.**

How could we figure out the radius once we know the diameter?

We can figure out the radius by dividing the diameter in half. The radius is one-half the measure of the diameter.

7 2 3.5

**The radius of the circle is 3.5 feet.**