Question

Cos2a=2cos^2a-1 for all of a true or false?

Answer 1

$\bf ~\hspace{10em}\textit{Double Angle Identities}\\\\ sin(2\theta)=2sin(\theta)cos(\theta) ~\hfill cos(2\theta)= \begin{cases} cos^2(\theta)-sin^2(\theta)\\ 1-2sin^2(\theta)\\ \boxed{\bf 2cos^2(\theta)-1} \end{cases} \\\\\\ tan(2\theta)=\cfrac{2tan(\theta)}{1-tan^2(\theta)}$

well, take a peek at the cos(2θ) identities.