Three points P(2x, x + 3), Q(0, x) and R(x + 3, x + 6) are collinear, then x is equal to

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