\(R = \sqrt {{a^2} +  {a^2} + 2a.a \cos 60^\circ } = a\sqrt 3 = 5\sqrt 3 \) એક્મ 

\(\tan \alpha = \frac{{a \sin 60^\circ }}{{a + \; a\cos 60^\circ }} = \frac{{\frac{{\sqrt 3 }}{2}}}{{\frac{3}{2}}} = \frac{1}{{\sqrt 3 }} = \tan 30^\circ \)

\( \therefore \alpha = {\text{30}}^\circ \)