[tex]\bf \textit{area of a sector of a circle}\\\\
A=\cfrac{\pi \theta r^2}{360}\qquad
\begin{cases}
\theta=\textit{angle in degrees}\\
r=radius\\
----------\\
\theta=90\\
A=36
\end{cases}\implies 36=\cfrac{90\cdot \pi \cdot r^2}{360}[/tex]
solve for "r"
