$2{\cos ^2}\theta - 2{\sin ^2}\theta = 1$, तो $\theta  =$ ..........$^o$ 

$\cos \frac{\pi }{7}\cos \frac{{2\pi }}{7}\cos \frac{{4\pi }}{7} = $

यदि $x\cos \theta = y\cos \,\left( {\theta + \frac{{2\pi }}{3}} \right) = z\cos \,\left( {\theta + \frac{{4\pi }}{3}} \right)$ , तब $\frac{1}{x} + \frac{1}{y} + \frac{1}{z}$ बराबर है

यदि $A + B + C = \pi ,$ तब  $\cos \,\,2A + \cos \,\,2B + \cos \,\,2C = $

$2\sin A{\cos ^3}A - 2{\sin ^3}A\cos A = $

$\tan 9^{\circ}-\tan 27^{\circ}-\tan 63^{\circ}+\tan 81^{\circ}$ का मान है________