\(OA=AB=OB=R\Rightarrow\Delta OAB\) đều \(\Rightarrow\widehat{ABO}=60^0\)
\(OB=BC=OC=R\Rightarrow\Delta OBC\) đều \(\Rightarrow\widehat{OBC}=60^0\)
\(\Rightarrow\widehat{ABC}=\widehat{ABO}+\widehat{OBC}=120^0\)
ABCM nội tiếp (O) \(\Rightarrow\widehat{AMC}=180^0-\widehat{ABC}=60^0\)
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