Answer
$\dfrac{2\sqrt 3-1}{2}$
Work Step by Step
Since, $\cos \dfrac{\pi}{6}=\dfrac{\sqrt 3}{2}$
Now, we have
$ f(\dfrac{\pi}{6})= 2 \cos \dfrac{\pi}{6}-\cos (2 \times \dfrac{\pi}{6}) $
or, $=2 \times \dfrac{\sqrt 3}{2}-\dfrac{1}{2}$
After simplifications, we have
or, $=\dfrac{2\sqrt 3-1}{2}$
