theorem :: FDIFF_10:3

for Z being open Subset of REAL st Z c= dom (cot * cot) holds
( cot * cot is_differentiable_on Z & ( for x being Real st x in Z holds
((cot * cot) `| Z) . x = (1 / ((sin . (cot . x)) ^2)) * (1 / ((sin . x) ^2)) ) )