theorem Th11: :: INTEGR14:11

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) ) )