Radius From Circumference

Part of the Circles section of Coddy's Geometry journey. Lesson 6 of 59.

A circle's circumference is 10π. Put it into the rule $C = 2\cdot \pi\cdot r$ .

$10\cdot \pi = 2\cdot \pi\cdot r$

Divide both sides by π: $10 = 2r$ . Then divide by 2: $r = 5$ .

When the circumference is a number times π, the π always cancels. The number in front of π is the diameter, here 10.

$Circle O with circumference 10 pi and radius 5$

Challenge

The circumference of circle O is 18π. Find the radius r.

Then press Check.

Try it yourself

$Use the figure to find r$

Use the figure to find r

All lessons in Circles

1Parts of a Circle

4Arcs and Sectors

7Angles in a Semicircle

10Tangents

2Circumference

5Radii Make Isosceles

8Angles on the Same Arc

11Circle Chases