This document discusses trigonometric double angle identities. It provides examples of using double angle identities to find expressions for sin(2θ) and cos(2θ) when given values for sinθ and cosθ. It also shows the identities that cos(θ - φ) = sinθsinφ + cosθcosφ, sin(θ + φ) + cos(θ + φ) = cosφ, and sin(θ + φ) - sin(θ - φ) = 2cosθsinφ.