Circles are a kind of curve that have a centre about which every single point is a certain distance (known as the radius) away from this centre. The equation of a circle with radius $r$ and centre $(a, b)$ is another form of Pythagoras’ theorem:

$$(x - a)^2 + (y - b) ^2 = r^2$$

Area

The area $A$ of a circle with radius $r$ can be calculated by:

$$A = \pi r^2$$

Perimeter

The perimeter $P$ of a circle is known as its circumference, and its equation is:

$$P = 2\pi r$$