Double Angle Formulas/Cosine

Theorem

 * $\cos 2 \theta = \cos^2 \theta - \sin^2 \theta$

where $\cos$ and $\sin$ denote cosine and sine respectively.

Also known as
Corollary $1$ and Corollary $2$ are sometimes known as Carnot's Formulas, for.

Also see

 * Double Angle Formula for Sine