The inscribed angle theorem

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• $\alpha$ - Central angle: an angle whose apex (vertex) is the center O of a circle and whose legs (sides) are radii intersecting the circle in two distinct points A and B
• $\beta$ - Inscribed Angle: the angle formed in the interior of a circle when two chords intersect on the circle

The inscribed angle theorem

The measure of the central angle is equal to twice the measure of the inscribed angle subtended by the same arc $\alpha =2\beta$

A central angle has countless inscribed angles. All of them are subject to the aforementioned rule: $\alpha =2\beta$