What Is Pi?

      Pi is a name for the special number 3.14159....

It was discovered over 2,000 years ago.

Early humans found that if you divide any circle's circumference by its diameter, you get a number close to 3.

As they got better at measuring, they found the number was closer to:

3.1

As they kept measuring more precisely, they found the number just kept going and going forever.

3.14159...

They called this number pi!

Tip: It's pi, not pie 🥧 😂.

      Pi is called an irrational number because its decimals keep going infinitely and never repeat.

3.141592653589793238...

Tip: For most math problems, 3.14 is all you need to remember.

      Pi is often written using the Greek letter for p, which is π. 

This symbol is what you'll use most when talking about pi.

How to Use Pi

      Use pi to find the circumference of a circle, once you know either the radius or diameter.

We just learned that π is defined as:

c/d = π
circumference ÷ diameter = ~3.14

We can rewrite this equation as:

c = πd
circumference = 3.14 × diameter

How did we do that?

We multiplied both sides by d, like this:

Tip: if you multiply both sides of an equation by the same variable or constant, the equation stays balanced.

This is a useful trick for moving stuff from one side of the equation to the other. 🧙‍♂️

Since diameter is just double the radius, we can replace d with 2r in our formula:

c = 2πr

      Circumference = 2 × π × radius.

Area of a Circle

      Pi can help you find the area of a circle, too.

The formula is a little harder:

Area of a circle = π × r × r

Tip: Mathematicians shorten r × r and just write r², pronounced "r squared".

Area of a circle = πr²

Tip: to square a number is to multiply it by itself.

Great job learning about pi!

Now, start the practice.