Through point C, which lies on the bisector of angle AOB, equal to 78

Through point C, which lies on the bisector of angle AOB, equal to 78, a straight line is drawn parallel to straight line AO. It intersects line OB at point D. Find the angles of triangle CDO.

OS is a bisector by condition, which means:
∠ AOC = COD = 78 ° / 2 = 39 °.
Consider the angles AOC and OCD. They are internal crosswise lying at parallel lines AO and CD, respectively, equal to each other (∠ AOC = ∠ OCD = 39 °)
Consider triangle CDO (isosceles, two angles are equal), find the angle at the vertex D:
∠ CDO = 180 ° – (39 ° + 39 °) = 180 ° – 78 ° = 102 °.
Answer: The angles of the CDO triangle are 39 °, 39 °, 102 °.