I'm having trouble trying to figure out how:
$\displaystyle \cos^22\theta$
turns into:
$\displaystyle 1/2 (1 + cos4\theta)$
Are we using some kind of trig identity?
Thanks!
Not sure what you're asking. The identity Also sprach Zarathustra mentioned is the power reduction identity.
$\displaystyle \cos^2 \,x = \dfrac{1}{2}(1 + \cos \,2x)$
Since you're looking for $\displaystyle \cos^2\,2\theta$, just plug in $\displaystyle 2\theta$ for $\displaystyle x$.