theorem :: FDIFF_9:36

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