If $$\sin x = {1 \over {\sqrt 5 }}$$, $$\sin y = {1 \over {\sqrt {10} }}$$, where $$0 < x < {\pi \over 2}$$, $$0 < y < {\pi \over 2}$$, then what is (x + y) equal to?

What is the principal value of $${\sin ^{ - 1}}\left( {\sin {{2\pi } \over 3}} \right)$$ ?

What is $${\tan ^{ - 1}}\left( {{1 \over 4}} \right) + {\tan ^{ - 1}}\left( {{3 \over 5}} \right)$$ equal to?

What is $$\tan \left\{ {2{{\tan }^{ - 1}}\left( {{1 \over 3}} \right)} \right\}$$ equal to?