half-angle formula The trigonometric functions of the half of an angle that are expressed in terms of separate functions of the angle. sin(x/2) = ±[(1 - cosx)/2]1/2 cos(x/2) = ±[(1 + cosx)/2]1/2 tan(x/2) = sinx/(1 + cosx) = (1 - cosx)/sinx

Related Terms: addition formula, double-angle formula