# Power Reduction Formulas/Cosine Squared/Proof 1

$\cos^2 x = \dfrac {1 + \cos 2 x} 2$
 $\displaystyle 2 \cos^2 x - 1$ $=$ $\displaystyle \cos 2 x$ Double Angle Formula for Cosine: Corollary 1 $\displaystyle \cos^2 x$ $=$ $\displaystyle \frac {1 + \cos 2 x} 2$ solving for $\cos^2 x$
$\blacksquare$