A function is said to be periodic if it satisfies with constant value
Period. The sine function and the cosine function are periodic, and both have the period of . That is,
Phase shift. The constant value is not the period for the sine and the cosine functions, but gives the following formulas of phase shift:
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.