[tex]\bf \textit{circumference of a circle}\\\\
C=2\pi r\qquad
\begin{cases}
r=radius\\
-----\\
C=36\pi
\end{cases}\implies 36\pi =2\pi r
\\\\\\
\cfrac{36\pi }{2\pi }=r\implies \boxed{18=r}\\\\
-------------------------------\\\\
\textit{area of a circle}\\\\
A=\pi r^2\qquad A=\pi \cdot \boxed{18}^2[/tex]
