theorem :: FDIFF_7:48

for Z being open Subset of REAL st Z c= dom ((#Z 2) * (sin / cos)) & ( for x being Real st x in Z holds
cos . x <> 0 ) holds
( (#Z 2) * (sin / cos) is_differentiable_on Z & ( for x being Real st x in Z holds
(((#Z 2) * (sin / cos)) `| Z) . x = (2 * (sin . x)) / ((cos . x) #Z 3) ) )