(b)
$\because 0 \leq \cos^{- 1}{x \leq \pi}$
And $0 < \cot^{- 1}{x < \pi}$
Given, $\lbrack\cot^{- 1}{x\rbrack + \lbrack\cot^{- 1}{x\rbrack = 0}}$
$\Rightarrow \lbrack\cot^{- 1}{x\rbrack = 0\ and\ \lbrack\cos^{- 1}{x\rbrack = 0}}$
$\Rightarrow 0 < \cot^{- 1}{x < 1\ and\ 0 \leq \cos^{- 1}{x < 1}}$
$\therefore x \in (\cot{1,\infty)\ and\ x \in (\cos{1,\ 1)}}$
$\Rightarrow x \in (\cot{1,\ 1})$