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Parts of a circle

The geometry of a circle, with circumference labelled

What is the circumference?

The circumference of a circle is the distance all the way around the circle. It’s the same as the perimeter, but a special word that we only use for the perimeter of a circle.

The circumference is always equal to twice the radius of the circle times π (pi), or 2πr.

Definition of a circle
What is the radius?
What is a perimeter?
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dinner plate

Dinner plate – can you figure out the circumference of a plate in your kitchen?

When did people figure this out?

Math teachers have been teaching their students how to figure out the diameter of a circle at least since the beginning of writing, about 3000 BCE, in Mesopotamia (in West Asia, in modern Iraq), and in Egypt, in Africa.

Math in ancient West Asia

How do we know this will work?

The African mathematician Euclid, working at the University of Alexandria in Egypt, proved that this is always true for any circle. You can follow his proof, which is pretty simple to understand.

Euclid’s proof
Who was Euclid?
More Egyptian math and science
African math

Some other ways this is useful

Once we know the circumference of a circle, we can also figure out the volume of a cylinder and the surface area of a cylinder.

What is a cylinder?

Learn by doing: figure out the circumference of a plate in your kitchen.
Area of a Circle
More about Geometry

Bibliography and further reading about circles:


More Geometry
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