Math Video Library Collection

Double-Number Identities E.g. cos 2A = cos(A + A) = cos A cos A – sin A sin A = cos² A – sin² A Other forms for cos 2A are obtained by substituting either cos² A = 1 – sin² A or sin² A = 1 – cos² A to get cos 2A = 1 – 2 sin² A or cos 2A = 2 cos² A – 1. Product-to-sum identities are used in calculus to find integrals of functions that are products of trigonometric functions.