Draw a semicircle and label the ends os the diameter as A and B, horizontally.
Label the center as O.
C and D divide the semicircle into three equal parts.
Therefore the three segments are OAD, ODC, CCB which are all equal.
The measure of the circular arc ADCB is 180°.
Therefore the measures of the central angles are
m∠AOD = m∠DOC = m∠COB = 60°.
The measures of the arc segments are
m-arc(AD) = m-arc(DC) = m-arc(CB) = 60°
