cho $\left | x \right |\leq 1$ và $\left | y \right |\leq 1$. Cmr
$\arccos x+\arccos y=\left\{\begin{matrix} \arccos \left ( xy-\sqrt{1-x^{2}} \sqrt{1-y^{2}} \right ); x+y\geq 0 & & \\ 2\pi -\arccos \left ( xy-\sqrt{1-x^{2}}\sqrt{1-y^{2}} \right ); x+y< 0& & \end{matrix}\right.$
