$$(2+\sqrt{3}); \cos\ 2\ \theta=1-\sin\ 2\ \theta$$
$\theta =\pi n_{1}+\frac{5\pi }{6}\text{, }n_{1}\in \mathrm{Z}$
$\theta =\pi n_{1}+\frac{\pi }{4}\text{, }n_{1}\in \mathrm{Z}$
