Does anyone know how to prove that oac and obc are congruent.

Hello,

The first step is to see that sides OA, OC, OB are all equal because they are the radii of the circle. Knowing that those sides are equal means that we have isosceles triangles. That gives us that the base angles are the same.

In OAC, we can subtract 118 from 180 to give us 62 degrees for the 2 bases angles. This would make each angle 31 degrees.

In OBC, the base angles are already 31, leaving the other angle 118.

There we have all the angles the same and 2 sides the same.

You could use SAS or ASA to prove that the triangles are congruent.

I hope this helps!
Good luck,
MrEquation