[tex]\textit{circumference of a circle}\\\\ C=2\pi r~~ \begin{cases} r=radius\\[-0.5em] \hrulefill\\ C=82.21 \end{cases}\implies 82.21=2\pi r\implies \cfrac{82.21}{2\pi }=r \\\\[-0.35em] ~\dotfill\\\\ \textit{area of a circle}\\\\ A=\pi r^2\qquad \qquad A=\pi \left( \cfrac{82.21}{2\pi } \right)^2\implies A=\cfrac{82.21^2}{4\pi }\implies A\approx 538.1[/tex]
