Double-Angle Formulas Formulas expressing trigonometric functions of an angle 2x in terms of functions of an angle x. sin(2x) = 2sin x cos x cos(2x) = cos2 x - sin2 x cos(2x) = 2 cos2 x - 1 cos(2x) = 1 - 2 sin2 x tan(2x) = (2tan x) / (1-tan2 x)
Formulas expressing trigonometric functions of an angle 2x in terms of functions of an angle x.