theorem :: FDIFF_9:40

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