### Video Summary:

The trigonometric functions, when defined for all real values,
are referred to as the circular functions. To define the
circular functions for any real number s, use the unit circle,
the circle with center at the origin and radius one unit. Start at
the point (1,0) and measure an arc of length s,
counterclockwise if s > 0 and clockwise if s < 0. Evaluating a Circular Function
Circular function values of real numbers are obtained in the same manner as trigonometric function values of angles measured in radians. This applies to both methods of finding exact values (such as reference angle analysis) and calculator approximations. Calculators must be in radian mode when finding circular function values.