Two-dimensional figure consisting of the set of all the points that are equally distant from the same point called the centre of the circle.

- the point
*O*is the centre of the circle, - \(\overline{\textrm{OE}}\) is a radius of the circle,
- \(\overline{\textrm{AD}}\) is a diameter of the circle,
- \(\overline{\textrm{FG}}\) is a chord of the circle,
- \(\overline{\textrm{TD}}\) is tangent to the circle at point D.

### Notation

Common notations for the circle are ** d** for the diameter,

*for the radius and*

**r****C**for the circumference. The letter

*O*refers to the centre of the circle.

### Formulas

The formulas used to calculate the circumference *C* and the area *A* of a circle are the following :

- \(C = 2π r\)
- \(A = πr^2\) or \(A = \dfrac{πd^2}{4}\)