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Given:
In the given circle O, BC is diameter, OA is radius, DC is a chord parallel to chord BA and .
To find:
The .
Solution:
If a transversal line intersect two parallel lines, then the alternate interior angles are congruent.
We have, DC is parallel to BA and BC is the transversal line.
[Alternate interior angles]
In triangle AOB, OA and OB are radii of the circle O. It means OA=OB and triangle AOB is an isosceles triangle.
We know that base angles of an isosceles triangle are congruent.
[Base angles of an isosceles triangle]
In triangle AOB,
Therefore, the measure of angle AOB is 120 degrees.