\[\begin{array}{1 1}(1)2\tan^{-1}\left( \begin{array}{c} 3 \\ 4 \end{array} \right)&(2)\tan^{-1}\left( \begin{array}{c} 4 \\ 3 \end{array} \right)\\(3)\frac{\pi}{2}&(4)\frac{\pi}{4}\end{array}\]