Albert Teen YOU ARE LEARNING:  Area of Circles  # Area of Circles ### Area of Circles

We can use a special constant called pi to find the area of a circle of any size.

Circles are quite special shapes, mathematically. Formulae for the perimeter and area make use of a number referred to as pi, which is a Greek letter pronounced “pie”.

The circumference of a circle is its outer edge. Each point on the circumference is equal distance from the centre. Let's refresh on a few key lines on a circle.

1

The radius is the length between the centre of the circle and any point on the circumference of the circle. 2

This is the diameter

The diameter crosses through the centre of the circle, and connects two points on the circumference. Therefore, it is also twice the radius. 3

Which line is this?  How much longer is the diameter of a circle compared to the radius? Pi is defined as the ratio between a circle's circumference to its diameter. This means that the circumference is pi times bigger than the diameter. We represent pi with the symbol $\pi$. Pi is: $3.14159265359$

1

We can use this formula to find the circumference

In this formula, $d$ is the diameter. 1

We can use the radius to find the circumference

Since the radius is twice the diameter, we can multiply the radius by 2 and multiply it by $\pi$ to find the circumference. 1

We can find the area of a circle using this formula

The $\pi$ relationship we have learned also allows us to find the area of a circle given its radius. 