[tex] \bf \textit{area of a sector of a circle}\\\\
A=\cfrac{\theta r^2}{2}~~
\begin{cases}
r=radius\\
\theta =angle~in\\
\qquad radians\\
------\\
r=5\\
\theta =\frac{\pi }{4}
\end{cases}\implies A=\cfrac{\frac{\pi }{4}\cdot 5^2}{2}\implies A=\cfrac{\quad \frac{25\pi }{4}\quad }{\frac{2}{1}}
\\\\\\
A=\cfrac{25\pi }{4}\cdot \cfrac{1}{2}\implies A=\cfrac{25\pi }{8} [/tex]
