Angles

Co-Terminal Angles

Co-terminal angles are angles with initial side on the positive x-axis, also called standard position, that have a common terminal side. For example 30^o, -330^o and 390^o are all co-terminal.  To find a angle's co-terminal angle, you just need to add or subtract 36o degrees, if the angle is measured in degrees, or  2 pi if the angle is measured in radians. 

Ex: Find a positive and a negative angle coterminal with a 55° angle.

55° – 360° = –305°

55° + 360° = 415°

A –305° angle and a 415° angle are coterminal with a 55° angle

Principal Angle

The principal angle is the least positive angle that a circle can provide. Always count not clock wise, therefore the principle angle is between 0 and 360. or 0 and 2 pi.

Reference Angle

The angle formed between the terminal arm and the x-axis. Also called the bow tie rule.