What Are Circles?

What do these things have in common?

They're all circles.

      A circle is a closed shape made of a perfectly curved line. 

A circle has a point inside it, called the center. 

Radius

      The distance from the center of a circle to any point on the edge is called the radius.

All points around the edge are the same distance from the center.

Diameter

      The diameter is the length of a straight line that goes through the center of a circle.

      To find the diameter, multiply the radius by 2.

radius × 2 = diameter

Circumference

      The distance around a circle is called the circumference.

Think of it as the length of the line that goes around a circle. 

The Circle and Pi

Circles have a special property. 

      If you divide the circumference by the diameter of any circle, you get a special number called pi.

pi = 3.141592654...

      The decimals in the value of pi just keep going on and on. It never ends, and the sequence of the digits never repeat.

Pi is a very important number. It helps us find the circumference of a circle.

Tip: you need to memorize at least the first 3 digits of pi, 3.14.

      Pi ≈ 3.14

We use pi so much, it gets it own symbol.

      The symbol for pi is this letter in the Greek alphabet: π.

      To find the circumference, multiply pi with the diameter.

circumference = π × diameter

Wow, pi is super useful.

Let's Review

      If you know the radius or diameter of a circle, you can use pi to figure out its circumference.

Excellent job learning about circles.

Now, complete the practice. It'll help you remember for longer.