Inscribed Angle Theorem

Theorem
An inscribed angle is equal to half the angle that is subtended by that arc.


 * InscribedAngleTheorem.png

Thus, in the figure above:
 * $\angle ABC = \frac 1 2 \angle ADC$


 * In a circle the angle at the center is double of the angle at the circumference, when the angles have the same circumference as base.