Circle Geometry

Inscribed angle: An angle made from points sitting on the circles circumference.

An inscribed angle is half the central angle.

When keeping the end points fixed, an inscribed angle is always the same, no matter where the vertex is.

The angle inscribed in a semicircle is always a right-angle.

A cyclic quadrilateral has four verticies which are all on a circles circumference. Opposite angles sum to 180.

A tangent forms a right-angle with a radius going to the same point on the circumference.