The radius of a circle is the distance from the center point to the edge of the circle. It's the same distance anywhere on the circle, because the circle has radial symmetry. So it doesn't matter where you measure the radius on the circle, and if you know one radius measurement for a circle, then you know all of them. Try it for yourself and see!

But suppose you *don't* know the radius of this circle? Suppose all you know is that the circumference is 30 centimeters?

One example would be if you were trying to measure a big tree. It would be pretty easy to take a rope and wrap it around the tree, and then measure the length of the rope to find out how big around the tree was (the circumference), but you'd have to cut the tree down to measure the radius. We don't want to cut this big tree down, so we'll need to use math to figure out the radius instead. Luckily we know that the radius of any circle is always the same as half of the circumference divided by π (pi). So the radius of this tree is 30/π = 9.55 centimeters, divided by two is 4.77 centimeters.

How can we be sure that this is true for every circle? Click here for a proof.

## More about GeometryMore Easy MathQuatr.us home

Professor Carr

Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University

Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.

Professor Carr's PSU page

Help support Quatr.us!

Quatr.us (formerly "History for Kids") is entirely supported by your generous donations and by our sponsors. Most donors give about \$10. Can you give \$10 today to keep this site running? Or give \$50 to sponsor a page?

• Christian Persecution
• Christian Empire

With the Presidential inauguration this weekend, it's a good time to review the Constitution, the Bill of Rights, and all the Constitutional amendments since the Bill of Rights. Also check out our articles on people who have been excluded from power in the United States - Native Americans, people of color, Mormons, Quakers, women...