theorem :: FDIFF_11:14

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