Explanation:
AB is a straight line. all angles around one point (here O) on one side of the line are 180°. after all, a line can be seen as the diameter of a circle, one side deals with a half-circle and therefore 180° (half of 360° of a full circle).
so, AOC and COB are supplementary angles (together they have 180°).
AOC + COB = 180° | dividing both sides by 2
AOC/2 + COB/2 = 180/2 = 90°
AOD = DOC = AOC/2
COE = EOB = COB/2
we can therefore put any of these bisected angles in place of the half main angles in the main equation :
DOC + COE = 90°
therefore, OD and OE enclose a right angle (90°).