\(\therefore T = mg\cos \theta + \frac{{m\,v_B^2}}{r} = 2\,mg\, ….(i)\)
\({v_B} = \sqrt {2gh} \)
\(\therefore {v_B} = \sqrt {2g\,r\cos \theta } \,\,\, ....(ii)\) \([h= r \,cos\theta ]\)
\(mg\cos \theta + \frac{m}{r}(2g\,r\,\cos \theta ) = 2\,mg\)
\(3mg\cos \theta = 2\,mg\)
\( \Rightarrow \cos \theta = \frac{2}{3}\)
\(\therefore \,\,\theta = {\cos ^{ - 1}}\left( {\frac{2}{3}} \right)\)