Circle
A circle is a simple closed shape in Euclidean geometry. A circle is set of all points in a plane that are at a given distance from the centre.
\begin{align} A &= \pi r^{2} = \frac {\pi d^{2}}{4}\\ \\ C &= 2\pi r = \pi d\\ \\ d &= 2r\\ \\ \pi &= 3.1415926535897...\\ \\ 2\pi &= 6.2831853071796...\\ \end{align}