If triangle ABC is equilateral, then its three angles are equal to 60°:
m <ACB = m <ABC = m <BAC = 60°
The arc AB is contained by the <ACB, the:
m <ACB=[m arc(AB)]/2
Replacing m<ACB by 60° in the equation above:
60°= [m arc(AB)] / 2
Solving for m arc(AB):
2*60°=2*[m arc(AB)]/2
120°= m arc(AB)
m arc(AB)=120°
Answer: m arc(AB)=120°
