If $A + B + C = \pi ,$ then ${\tan ^2}\frac{A}{2} + {\tan ^2}\frac{B}{2} + $${\tan ^2}\frac{C}{2}$ is always
$ \le 1$
$ \ge 1$
$= 0$
$= 1$
Prove that: $\cos 6 x=32 x \cos ^{6} x-48 \cos ^{4} x+18 \cos ^{2} x-1$
If $\sin x + \cos x = \frac{1}{5},$ then $\tan 2x$ is
$\tan \frac{A}{2}$ is equal to
If $A + B + C = \pi \,(A,B,C > 0)$ and the angle $C$ is obtuse then
If $2\tan A = 3\tan B,$ then $\frac{{\sin 2B}}{{5 - \cos 2B}}$ is equal to