If `f(x) = cos x cos 2x cos 4x cos (8x). cos 16x` then find `f' (pi/4)`

Explanation

`f(x)=(2 sin x cdotcos x cdot cos 2x cdot cos 4x cdot cos 8x cdot cos 16x)/(2 sin x)`
`=(sin 32 x)/(2^(5) sin x)`
`therefore" "f'(x)=(1)/(32)xx(32 cos 32 x xxsin x - cos x xx sin 32 x)/(sin^(2)x)`
`therefore" "f'((pi)/(4))=(32xx(1)/(sqrt(2))-(1)/sqrt(2)xx0)/(32xx((1)/(sqrt(2)))^(2))=sqrt(2)`