u ' = [ x /( x +1 ) ] ' = 1 / ( x+ 1 ) ²
u² + 1 = [ x /(x +1 ) ]² + 1 = ( 2x² + 2x +1 ) / ( x +1 ) ²
(arctg u ) ' = u ' / (u² +1 )
u ' 1 ( x +1 ) ²
(arctg u) ' = -------------- = ---------------------- · --------------------
u² +1 ( x +1 )² 2x² +2x +1
= 1 / ( 2x² +2x +1 )
