4cos2x = - √8
cos(3x + 180º) = √3/2
cos2x-sin2xsecx = 1
Thanks in advance.
Some identities could be helpful here, but maybe you don't need them at all.
Take #1 for example
$\displaystyle \displaystyle 4\cos 2x = -\sqrt{8}$
$\displaystyle \displaystyle \cos 2x = \frac{-\sqrt{8}}{4}$
$\displaystyle \displaystyle \cos 2x = \frac{-2\sqrt{2}}{4}$
$\displaystyle \displaystyle \cos 2x = \frac{-\sqrt{2}}{2}$
$\displaystyle \displaystyle \cos 2x = \frac{-1}{\sqrt{2}}$
$\displaystyle \displaystyle 2x = \frac{3\pi}{4}, \frac{5\pi}{4}$
$\displaystyle \displaystyle x = \frac{3\pi}{8}, \frac{5\pi}{8}$