theorem Th95: :: SIN_COS9:95

for Z being open Subset of REAL st Z c= ].(- 1),1.[ holds
( (id Z) (#) arctan is_differentiable_on Z & ( for x being Real st x in Z holds
(((id Z) (#) arctan) `| Z) . x = (arctan . x) + (x / (1 + (x ^2))) ) )