${\rm{cosec }}A - 2\cot 2A\cos A = $
$2\sin A$
$\sec A$
$2\cos A\cot A$
None of these
If a $cos^3 \alpha + 3a \,cos\, \alpha \, sin^2\, \alpha = m$ and $asin^3\, \alpha + 3a \, cos^2\, \alpha \,sin\, \alpha = n$ . Then $(m + n)^{2/3} + (m - n)^{2/3}$ is equal to :
If $\sin \left( {x + \frac{{4\pi }}{9}} \right) = a;\,$ $\frac{\pi }{9}\, < \,x\, < \,\frac{\pi }{3},$ then $\cos \left( {x + \frac{{7\pi }}{9}} \right)$ equals :-
If $A + B + C = \pi ,$ then ${\tan ^2}\frac{A}{2} + {\tan ^2}\frac{B}{2} + $${\tan ^2}\frac{C}{2}$ is always
$\cos \frac{\pi }{5}\cos \frac{{2\pi }}{5}\cos \frac{{4\pi }}{5}\cos \frac{{8\pi }}{5} = $
The expression $\frac{{\cos 6x + 6\cos 4x + 15\cos 2x + 10}}{{\cos 5x + 5\cos 3x + 10\cos x}}$ is equal to