# Circles

A circle is a shape on a plane in which all points are equally distant from the center.

## Terms and Symbols Related to Circles

Term | Symbol | Definition |
---|---|---|

Area | $a$ | The amount of space occupied by the surface of the circle |

Circumference | $c$ | The perimeter of a circle |

Diameter | $d$ | The distance of a straight line from one point on a circle to the opposite point on the circle and passes through the center |

Pi | $\pi$ | The ratio of a circle's circumference to its diameter. Equal to about $22/7$ or $3.14$ |

Radius | $r$ | The distance from the center to a point on the edge of a circle |

## Formulas to Find Properties of Circles

### Area of a Circle

$a = \pi r^2$

$a = \pi (d/2)^2$

### Circumference of a Circle

$c = 2 \pi r$

$c = d \pi$

### Diameter of a Circle

$d = 2r$

$d = c/\pi$

### Radius of a Circle

$r = d / 2$

$r = c/2\pi$

## Deeper Knowledge on Circles (Geometry)

### Radian

The angle which subtends an arc equal to the length of the radius of a circle

## Broader Topics Related to Circles (Geometry)

### Geometry

The mathematics of shapes