theorem :: FDIFF_11:13

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 . (arctan . x)) / (1 + (x ^2)) ) )