This document discusses trigonometric identities. It includes: - Pythagorean identities relating sine, cosine, and their reciprocals - Tangent/cotangent identities relating tangent and cotangent to sine and cosine - Fundamental trig identities for sine, tangent, cotangent, and secant - Examples of simplifying trig expressions using identities and of proving trig identities by manipulating expressions until they are equal.