If $\alpha $ and $\beta $ are solutions of $sin^2\,x + a\, sin\, x + b = 0$ as well that of $cos^2\,x + c\, cos\, x + d = 0$ , then $sin\,(\alpha + \beta )$ is equal to
$\frac{{2bd}}{{{b^2} + {d^2}}}$
$\frac{{{a^2} + {c^2}}}{{2ac}}$
$\frac{{{b^2} + {d^2}}}{{2bd}}$
$\frac{{2ac}}{{{a^2} + {c^2}}}$
$\frac{{\sec \,8\theta - 1}}{{\sec \,4\theta - 1}}$ is equal to
Prove that $\frac{\cos 4 x+\cos 3 x+\cos 2 x}{\sin 4 x+\sin 3 x+\sin 2 x}=\cot 3 x$
$\cos \frac{\pi }{7}\cos \frac{{2\pi }}{7}\cos \frac{{4\pi }}{7} = $
Prove that $=\frac{\sin 5 x-2 \sin 3 x+\sin x}{\cos 5 x-\cos x}=\tan x$
The value of $\cos \left(\frac{2 \pi}{7}\right)+\cos \left(\frac{4 \pi}{7}\right)+\cos \left(\frac{6 \pi}{7}\right)$ is equal to