theorem :: FDIFF_10:1

for Z being open Subset of REAL st Z c= dom (tan * cot) holds
( tan * cot is_differentiable_on Z & ( for x being Real st x in Z holds
((tan * cot) `| Z) . x = (1 / ((cos . (cot . x)) ^2)) * (- (1 / ((sin . x) ^2))) ) )