Respuesta :
[tex] \bf \textit{area of a circle}\\\\
A=\pi r^2~~
\begin{cases}
r=radius\\[-0.5em]
\hrulefill\\
A=78.5
\end{cases}\implies 78.5=\pi r^2\implies \cfrac{78.5}{\pi }=r^2
\\\\\\
\sqrt{\cfrac{78.5}{\pi }}=r\implies 4.99873244587\approx r [/tex]
