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

$a = \pi r^2$

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

$c = 2 \pi r$

$c = d \pi$

$d = 2r$

$d = c/\pi$

$r = d / 2$

$r = c/2\pi$

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

The mathematics of shapes