$ \hat{M} = \hat{O}  $ $ \hat{M}=\frac{\stackrel\frown {AB}}{2}$ $\hat{O}=\stackrel\frown {AB}$ $\stackrel\frown {AB}+\frac{\stackrel\frown {AB}}{2}=360  \to \frac{\stackrel\frown {3AB}}{2}=360\stackrel\frown {AB}=240$ $ \hat{M}=\frac{\stackrel\frown {AB}}{2}=240\div2=120^\circ$