An angle whose vertex is on a circle and whose two sides intersect the circle. It has the following properties: (1) The circumferential angle of an arc is equal to half the central angle it faces; (2) The degree of the circumferential angle is equal to half of the radian it subtends; (3) In the same circle or equal circle, the circumferential angles of the same arc or equal arc are equal; Equal circumferential angles face equal arcs.

The vertex is on the circle, and the angle at which both sides intersect the circle is called the circumferential angle.

Inference 1 is equal to the circumferential angle of the same arc or equal arc; Equal circle in the same circle or equal circle.

The arcs subtended by these angles are also equal.

Inference 2: the circumference angle (or diameter) of a semicircle is a right angle; A 90-degree circumferential angle.

A chord is a diameter.

Inference 3 If the median line of one side of a triangle is equal to half of this side, then this triangle

This is a right triangle.