A function is said to be *periodic*
if it satisfies with constant value

(2-1) |

The least such positive value for which (2-1) holds is called the

(2-2) |

**Period.**
The sine function and the cosine function
are periodic, and both have
the period of .
That is,

and | (2-3) |

(2-2) and (2-3) together implies that we have for any integer ,

and

**Phase shift.**
The constant value is not the period for the sine and the cosine
functions, but gives the following formulas of phase shift:

and

**Values of sine and cosine functions.**
Use the ``reference angle ''
(the acute angle between the terminal side
of the original angle and the -axis)
to find the coordinate
on the unit circle.

Department of Mathematics