An important word that is used in circle theorems is subtend . An angle is created by two lines. The angle in between the two lines is subtended by the arc between C and D.

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. Angle ...