[tex]\it m(\widehat{AOB})=m(\stackrel\frown{AB})=60^o\ \ \ \ \ (1)\\ \\ OA=OB=8\ cm\ (raze)\ \Rightarrow\ \Delta AOB\ -\ isoscel\ \ \ \ \ \ (2)\\ \\ (1),\ (2)\ \Rightarrow\ \Delta AOB\ -\ echilateral\ \Rightarrow\ AB=OA=OB=8\ cm\\ \\ \mathcal{P}=3\cdot\ell=3\cdot8=24\ cm[/tex]