theorem :: DIFF_3:94

for h, x being Real
for f being Function of REAL,REAL st ( for x being Real holds f . x = (tan (#) tan) . x ) & x in dom tan & x + h in dom tan holds
(fD (f,h)) . x = - (((1 / 2) * ((cos (2 * (x + h))) - (cos (2 * x)))) / (((cos (x + h)) * (cos x)) ^2))