theorem :: FDIFF_10:14

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