Ta có $\rm \dfrac{\pi}{12}=\dfrac{\pi}{3}-\dfrac{\pi}{4}$ 

`->`$\rm cos(\dfrac{\pi}{12})=cos(\dfrac{\pi}{3}-\dfrac{\pi}{4})$ 

$\rm =cos(\dfrac{\pi}{3}).cos(\dfrac{\pi}{4}) +sin(\dfrac{\pi}{3}) .sin(\dfrac{\pi}{4})$

$\rm =\dfrac{1}{2}.\dfrac{\sqrt{2}}{2} + \dfrac{\sqrt{3}}{2}.\dfrac{\sqrt{2}}{2}$ 

$\rm =\dfrac{\sqrt{2}+\sqrt{6}}{4}$