$f(x)=(\cos x) (\cos 2x).........( \cos nx) => f'(x)+ \sum \limits _{r=1} ^ n (r \tan rx) f(x)=$

Question

\[\begin {array} {1 1} (1)\;f(x) & \quad (2)\;0 \\ (3)\;-f(x) & \quad (4)\;2f(x) \end {array}\]