${\sin ^{ - 1}}\left( {{{\sqrt 3 } \over 2}} \right) + {\cos ^{ - 1}}\left( {{{ - \sqrt 3 } \over 2}} \right) + {\tan ^{ - 1}}\left( { - 1} \right)$ $= {\pi \over 3} + {{5\pi } \over 6} - {\pi \over 4}$ $= {{4\pi + 10\pi - 3\pi } \over {12}} = {{11\pi } \over {12}}$