The measure of an arc formed by two adjacent arcs is the sum of the measures of the two arcs.
In the diagram above, the following relation holds true.
Start by drawing the radii PA, PB, and PC and labeling the central angles.
By definition, the arc measure is equal to the measure of the related central angle. Then, mAB=m∠1 and mBC=m∠2. Finally, by the Angle Addition Postulate, it is obtained the desired result.