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{{ printedBook.courseTrack.name }} {{ printedBook.name }} 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.

$mABC=mAB+mBC$

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.

$mABC=m∠APCm∠1+m∠2⇓mABC=mAB+mBC $