Differentiate the functions given in w.r.t. x.

cos x . cos 2x . cos 3x

Given: cos x . cos 2x . cos 3x

Let y= cos x . cos 2x . cos 3x

Taking log on both sides, we get

log y = log(cos x . cos 2x . cos 3x)

⇒log y = log(cos x) + log(cos 2x) + log(cos 3x)

Now, differentiate both sides with respect to x