A **circle** is a shape that is round.

It has infinite lines of symmetry through the centre of the circle.

## FormulaeEdit

To find the area and circumference of a circle, use these formulae:

$ \pi r^2 $ (for area)

$ 2\pi r $ (for circumference)

## Circle TheoremsEdit

A circle has many relationships between its angles.

### Angles Subtended on the Same ArcEdit

Angles formed from two points on the circumference are equal to other angles, in the same arc, formed from those two points.

### Angle in a Semi-CircleEdit

Angles formed by drawing lines from the ends of the diameter of a circle to its circumference form a right angle. So c is a right angle.

### Angle at centreEdit

The angle formed at the centre of the circle by lines originating from two points on the circle's circumference is double the angle formed on the circumference of the circle by lines originating from the same points. i.e. a = 2b.

## SectorEdit

The area of a sector = $ \frac {\theta}{360} \pi r^2 $