This document provides rules and formulae for double angles in trigonometry. It contains: 1) Formulae for sin(A+B), cos(A+B), sin2A, and cos2A in terms of sinA, cosA, sinB, and cosB. 2) Examples of using the double angle formulae to find sin2A and cos2A when given sinA and cosA. 3) Alternative versions of the cos2A formula as cos2A = 1 - 2sin2A or cos2A = 2cos2A - 1, which are useful for trigonometric equations.