More simply, the angle at the centre is double the angle at the circumference. Angle OGH (\(y\)) = angle OHG because triangle GOH is also isosceles. Lengths OH and OG are also both radii.

therefore this forms an isosceles triangle inside the circle. Also, note that since triangle AOB is isosceles, then \(\angle OAB=\angle OBA\). \(\angle AOC\) is a straight angle, so will add up to ...