theorem :: FDIFF_11:27

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