so, is a semi-circle, half a circle, recall a circle has a total of 360°, so half of that will be 180°.
the diameter of that circle is 10, so its radius is half that, or 5.
[tex]\bf \textit{arc's length}\\\\ s=\cfrac{\theta \pi r}{180}~~ \begin{cases} r=radius\\ \theta =angle~in\\ \qquad degrees\\[-0.5em] \hrulefill\\ \theta =180\\ r=5 \end{cases}\implies s=\cfrac{(180)(\pi )(5)}{180}\implies s=5\pi \stackrel{\pi =3.14}{\implies s=15.7}[/tex]
