Let $$\overrightarrow a = \widehat i + \widehat j + \widehat k,\,\overrightarrow b = \widehat i - \widehat j + \widehat k$$ and $$\overrightarrow c = \widehat i - \widehat j - \widehat k$$ be three vectors. A vector $$\overrightarrow v $$ in the plane of $$\overrightarrow a $$ and $$\overrightarrow b ,$$ whose projection on $$\overrightarrow c $$ is $${{1 \over {\sqrt 3 }}}$$ , is given by

The vector (s) which is/are coplanar with vectors $${\widehat i + \widehat j + 2\widehat k}$$ and $${\widehat i + 2\widehat j + \widehat k,}$$ and perpendicular to the vector $${\widehat i + \widehat j + \widehat k}$$ is/are

Let $$f\left( \theta \right) = \sin \left( {{{\tan }^{ - 1}}\left( {{{\sin \theta } \over {\sqrt {\cos 2\theta } }}} \right)} \right),$$ where $$ - {\pi \over 4} < \theta < {\pi \over 4}.$$

Then the value of $${d \over {d\left( {\tan \theta } \right)}}\left( {f\left( \theta \right)} \right)$$ is