Identities

tan t =	sin t cos t		cot t =	1 tan t	=	cos t sin t
sec t =	1 cos t		csc t =	1 sin t

The Pythagorean formula for sines and cosines.

sin² t + cos² t = 1

Identities for negative angles. Sine, tangent, cotangent, and cosecant are odd functions while cosine and secant are even functions.

sin –t = –sin t

cos –t = cos t

tan –t = –tan t

Sum formulas for sine and cosine

sin (s + t) = sin s cos t + cos s sin t

cos (s + t) = cos s cos t – sin s sin t

Double angle formulas for sine and cosine

sin 2t = 2 sin t cos t

cos 2t = cos2 t – sin2 t = 2 cos2 t – 1 = 1 – 2 sin2 t

Example:

http://www.youtube.com/watch?v=SZu_EVV4jjY