Key learning points

1. There is a multiplicative relationship between the radius and diameter of a circle.
2. The ratio between the diameter and circumference of a circle is always the same.
3. That ratio is called π and it is an irrational number.
4. The formula for the circumference of a circle is C = πd.
5. Using the definition of the diameter you can also write this as C = 2πr.

Keywords

• Diameter - The diameter of a circle is any line segment that starts and ends on the edge of the circle and passes through the centre.

• Radius - The radius of a circle is any line segment that joins the centre of a circle to its edge.

• Circumference - The circumference of a circle is the perimeter of the circle.

Common misconception