theorem :: FDIFF_11:25

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