[tex]\bf \textit{arc's length}\\\\
s=r\theta \quad
\begin{cases}
r=radius\\
\theta =angle~in\\
\qquad radians\\
------\\
s=10\\
\theta =\frac{2\pi }{3}
\end{cases}\implies 10=r\cdot \cfrac{2\pi }{3}\implies \cfrac{30}{2\pi }=r\implies \cfrac{15}{\pi }=r[/tex]
