+128408 Triangles AOB and DOC are congruent
Then arc BC = arc AD =108°
And angle CAB (Angle A) intercepts arc BC.....and it is an inscribed angle, so it has 1/2 the measure of this arc = 54°
And angle B has the same measure since OA = OB
So angle AOB = 180 - 2(54) = 180 - 108 = 72°
