[tex]\bf \textit{area of a circle}\\\\
A=\pi r^2~~
\begin{cases}
r=radius\\
-----\\
A=4\pi
\end{cases}\implies 4\pi =\pi r^2
\\\\\\
\cfrac{4\pi }{\pi }=r^2\implies 4=r^2\implies \sqrt{4}=r\implies \boxed{2=r}
\\\\\\
\textit{circumference of a circle}\\\\
C=2\pi r\qquad \qquad \implies C=2\pi \left( \boxed{2} \right)\implies C=4\pi [/tex]
