$\cos 20^\circ \cos 40^\circ \cos 80^\circ = $
$1/2$
$1/4$
$1/6$
$1/8$
${\sin ^4}\frac{\pi }{8} + {\sin ^4}\frac{{3\pi }}{8} + {\sin ^4}\frac{{5\pi }}{8} + {\sin ^4}\frac{{7\pi }}{8} = $
$\sqrt 2 + \sqrt 3 + \sqrt 4 + \sqrt 6 $ is equal to
If $\tan \frac{\theta }{2} = t,$then $\frac{{1 - {t^2}}}{{1 + {t^2}}}$is equal to
$\tan 3A - \tan 2A - \tan A = $
If $\theta $ is an acute angle and $\sin \frac{\theta }{2} = \sqrt {\frac{{x - 1}}{{2x}}} $, then $\tan \theta $ is equal to