Half-Angle and Double Angle Formulas
Using the addition formulas we've learned, we can develop new identities for sin and cos that can be used to express the sine and cosine of half-angles in terms of the cosine of a whole angle. Recall the addition formula:
(1) - cos (a + b) = (cos a)(cos b) – (sin a)(sin b)
Simplify.
Using the identity (sin a)2 + (cos a)2 = 1, replace 
in (2):
