Double Angle Formula for Cosine/Corollary 1

Corollary to Double Angle Formula for Cosine

 * $\cos 2 \theta = 2 \ \cos^2 \theta - 1$

where $\cos$ denotes cosine.

Also see

 * Square of Cosine: $\cos^2 \theta = \dfrac {1 + \cos 2 \theta} 2$