theorem Th9: :: INTEGR14:9

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) ) )