$f\left( x \right) = 3\left( {{{\sin }^4}x + {{\cos }^4}x} \right)$ $- 2\left( {{{\sin }^6}x + {{\cos }^6}x} \right)$

$f\left( x \right) = {\cos ^6}x + 2{\sin ^4}x{\cos ^2}x$ $+ 3{\sin ^2}x{\cos ^4}x + {\sin ^4}x$

$f\left( x \right) = \cos \left( {x - {\pi  \over 3}} \right)\cos \left( {x + {\pi  \over 4}} \right)$ $+ \cos \left( {x + {\pi  \over 6}} \right)\cos \left( {x + {{3\pi } \over 4}} \right)$

$f\left( x \right) = {\cos ^2}x + {\cos ^2}\left( {{{2\pi } \over 3} + x} \right) + {\cos ^2}\left( {{{2\pi } \over 3} - x} \right).$