Complete set of values of x in   satisfying the inequality   is

  …..(1) 
Necessary condition
$\left.\begin{array}{l}\cos \text{x}<0\\ \cos 3\text{x}>0\end{array}\right\}\Rightarrow \text{x}\in \left(\frac{\pi }{2},\frac{5\pi }{6}\right)$ 
Now, (1)