This activity is about finding the circumferences of circles.

The circumference of a circle is its perimeter or the distance around its edge.

You need to learn these two important formulae for a circle of radius, **r** or diameter, **d**:

Circumference of a circle = 2πr

Circumference of a circle = πd

This is because the diameter of a circle is two times the length of the radius: **2 x r = d**

**The value of ****π **(pi) is 3.142.... but is an infinite number, so the best way to do any calculations with pi is to use a scientific calculator that has a special **π ** button.

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**Example 1**

Find the circumference of a circle of radius 6 cm.

**Answer**

**Circumference = 2 × π × r **= 2 × π × 6 = 12 × π = **37.7 **cm (1 decimal place)

**Example 2**

Find the circumference of a circle of diameter 6 cm.

**Answer**

**Circumference = π × d **= π × 6 = **18.8 **cm (1 decimal place)

Use a scientific calculator to work out your answer because you can have a more accurate number for pi (**π **).

If you don't have one in the house, you can access one online.

Let's have a go at some questions.