延长CM,交OB于点D.
因为,DM∥OA,AM = BM ,
所以,∠AOC = ∠OCD ,OD = BD = ½OB = ½OC ,
可得：∠AOC = ∠OCD = 30° = (1/3)∠AOB ,
所以,弧AC = (1/3)弧AB .