Double Angle Formula for Cosine/Corollary 4

Corollary to Double Angle Formula for Cosine

 * $1 + \cos \theta = 2 \cos^2 \dfrac \theta 2$

Proof
From Double Angle Formula for Cosine: Corollary 1:
 * $\cos 2 \theta = 2 \cos^2 \theta - 1$

The result follows from substituting $\dfrac \theta 2$ for $\theta$ and rearranging.