theorem :: FDIFF_8:13

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