# Circles

## Key Facts

You must learn the following:

- The equation of a circle with centre
and radius is given by - If the equation is given in expanded form, eg.
, then it is necessary to **complete the square**before identifying the centre and radius:

## Test Yourself - Equation of a Circle

## Relevant Circle Theorems

**tangent**to the circle will meet the

**radius**at a right-angle. (Gradient of radius is usually calculated easily from a known point on the circle, P, and the centre of the circle, C).

**perpendicularly bisected**by a specific radius.

Download our apps here: