Question

If $\theta =\frac{\pi }{8}$, then the value of $\cos 7\theta +\cos \theta$ is

Answer 1

$\cos 7\theta +\cos \theta =\cos \left(\frac{7\pi }{8}\right)+\cos \left(\frac{\pi }{8}\right)=\cos (\pi -\frac{\pi }{8})+\cos \left(\frac{\pi }{8}\right)=-\cos \left(\frac{\pi }{8}\right)+\cos \left(\frac{\pi }{8}\right)=0$