recall, the Unit Circle is called like so because it has a radius of 1.
so, what's the arc for an angle of 2π/3 and a radius of 1?
[tex]\bf \textit{arc's length}\\\\
s=r\theta \quad
\begin{cases}
r=radius\\
\theta =angle~in\\
\qquad radians\\
------\\
r=1\\
\theta = \frac{2\pi }{3}
\end{cases}\implies s=1\cdot \frac{2\pi }{3}\implies s=\frac{2\pi }{3}[/tex]
