[tex]\it b= \dfrac{\pi}{3}-a[/tex]
[tex]\it cos b= cos(\dfrac{\pi}{3}-a) =cos \dfrac{\pi}{3}cos a + \sin\dfrac{\pi}{3} \it\sin a \ \Longrightarrow[/tex]
[tex]\it {\Longrightarrow cos b = \dfrac{1}{2}cos a +\dfrac{\sqrt3}{2} sin a \ |_{\cdot2}\Longrightarrow 2b = cos a + \sqrt3 sin a}\ .[/tex]
