The principal value of $${\cos ^{ - 1}}\left( {{1 \over 2}} \right) + {\sin ^{ - 1}}\left( { - {1 \over {\sqrt 2 }}} \right)$$ is

If $${({x^2} + {y^2})^2} = xy$$, then $${{dy} \over {dx}}$$ is

If a matrix A is both symmetric and skew symmetric, then A is necessarily a

Let set X = {1, 2, 3} and a relation R is defined in X as : R = {(1, 3), (2, 2), (3, 2)}, then minimum ordered pairs which should be added in relation R to make it reflexive and symmetric are