$\alpha = \frac{\pi }{2}{\sin ^2}\theta \Rightarrow \tan \alpha + \cot \left( {\frac{\pi }{2} - a} \right) = 2\sqrt 3 \Rightarrow 2\tan \alpha = 2\sqrt 3 $
$ \Rightarrow \tan \alpha = \sqrt 3 \xrightarrow{{0 < \alpha < \frac{\pi }{2}}}\alpha = \frac{\pi }{3} \Rightarrow \frac{\pi }{2}{\sin ^2}\theta = \frac{\pi }{3} \Rightarrow {\sin ^2}\theta = \frac{2}{3}$
$ \Rightarrow {\tan ^2}\theta = 2,{\cot ^2}\theta = \frac{1}{2} \Rightarrow \sin \left( {5\frac{\pi }{3} \times 2} \right) + \cos \left( {\frac{{5x}}{3} \times \frac{1}{2}} \right) = $
$\sin \left( {3x + \frac{\pi }{3}} \right) + \cos \left( {\pi - \frac{\pi }{6}} \right) = - \sin \frac{\pi }{3} - \cos \frac{\pi }{6} = - \frac{{\sqrt 3 }}{2} - \frac{{\sqrt 3 }}{2} = - \sqrt 3 $