What Is a Circle?
Before we dive into formulas, let's make sure we know the key parts of a circle.
A circle is a perfectly round shape where every point on the edge is exactly the same distance from the centre.
The exact middle point of a circle.
The distance from the centre of a circle to any point on its edge. Often written as .
The distance across a circle through its centre. It is exactly twice the radius: .
The total distance around the outside of a circle — its perimeter.
A straight line segment connecting any two points on the edge of a circle. The diameter is a special chord that passes through the centre.
Think of a radius like the spoke of a bicycle wheel — it goes from the hub (centre) to the rim (edge). The diameter is like a spoke that passes all the way through the hub to the other side.
Investigating Pi (π)
One of the most fascinating numbers in all of mathematics comes from circles. If you take any circle — a coin, a pizza, the Earth — and divide its circumference by its diameter, you always get the same number.
The ratio of a circle's circumference to its diameter. It is an irrational number, meaning its decimal digits go on forever without repeating. Its approximate value is
For most MYP calculations, you can use or the button on your calculator.
is irrational — it cannot be written as a simple fraction. The fraction is a useful approximation but is not exactly equal to .
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