, and are point on a circle centred ;
and are tangents to the circle;
and are chords in the circle;
The reflex angle AOC is equal to , because the angle subtended by an arc at the centre is twice the angle subtended at the circumference.
So the obtuse angle AOC is then 130° as angles around a point sum to 360°.
The quadrilateral ABCO has internal angles at A and C of 90°, because tangents and radii meet at 90°.