The inscribed angle is always half the central angle (and half the intercepted arc). Give any one of the three and find the others — with a color-coded diagram.
Inscribed angle = ½ × intercepted arc
Inscribed angle = ½ × central angle
Central angle = intercepted arc (in degrees)
When the intercepted arc is a semicircle (180°), the inscribed angle is always exactly 90°. Any angle inscribed in a semicircle is a right angle.
A tutor can explain why the inscribed angle theorem is true — and connect it to arcs, chords, and all the other circle relationships.