[tex]\textit{circumference of a semicircle}\\\\ C=\pi r ~~ \begin{cases} r=radius\\[-0.5em] \hrulefill\\ C=81.212 \end{cases}\implies 81.212=\pi r\implies \cfrac{81.212}{\pi }=r \\\\\\ \stackrel{\textit{its diameter is}}{2r\implies 2\left( \cfrac{81.212}{\pi } \right)} ~~ \approx ~~ 51.7[/tex]
