If f(x) = cos x cos 2x cos 4x cos (8x). ...

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

A

`sqrt2`

B

`1/sqrt2`

C

1

D

`(-1)/sqrt2`

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The correct Answer is:

A

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