If $f(x)=\cos x \cos 2 x \cos 4 x \cos 8 x$, then $f^{\prime}\left(\frac{\pi}{4}\right)$ is

Question

Professor PC Thomas Classes · Answer

$f(x) = \frac{\sin(16x)}{16\sin x}$. Differentiating and substituting $\pi/4$ yields $\sqrt{2}$.