Definition: An angle is a way of measuring a part of a circle. Angles are measured in degrees with the symbol °. More
An angle marks off a section of a circle by extending two rays from the center of the circle.
 The center of the circle is the vertex of the angle, the two rays are the legs of the angle.
 If the two rays form a straight line, they divide the circle in half and the angle is 180°.
 If the two rays mark off a quarter of a circle, the angle is 90° and is called a "right" angle.
 Angles are also measured in radians (no unit) where 1 ≈ 57° and 180° = π ≈ 3.14 .
Interactivity
 Click and drag the point С to change the angle <ABC .
Regulations: Angle <ABC  basics
 An angle of 90° is a fourth of a circle.
 An angle of 90° is called a right angle.
 An angle of 90° is marked by a square (make an angle of 90° in the interactivity below!).
 An angle of 180° is a half of a circle.
 An angle of 180° is the line segment \overline{CA} where the point B is the midpoint.
 An angle of 360° is the whole circle.
 An angle of 360° is the line segment \overline{BA} and the points А and С overlap.
 An angle of 360° cannot be draw in the interactivity (it automatically reverts to the coterminal angle 0°).
