Official solution
\[y = {\tan ^{ - 1}}\left( {{{\tan x - 1} \over {\tan x + 1}}} \right)\]
\[ \Rightarrow \] \[ - {\tan ^{ - 1}}\left( {{{1 - \tan x} \over {1 + \tan x}}} \right)\]
\[ \Rightarrow \] \[ - {\tan ^{ - 1}}\left( {\tan \left( {{\pi \over 4} - x} \right)} \right)\]
\[ \Rightarrow \] \[ - \left( {{\pi \over 4} - x} \right)\]
\[ \Rightarrow \] \[{{dy} \over {dx}} = 1\]
Now if differentiation of \[{x \over 2}\] w.r.t is \[{1 \over 2}\]
\[ \Rightarrow \] So differentiation of y w.r.t \[{x \over 2}\] is \[1 \over {1 \over 2}\] = 2