theorem :: DIFF_3:66

for h, x being Real
for f being Function of REAL,REAL st ( for x being Real holds f . x = 1 / (cos x) ) & cos x <> 0 & cos (x + h) <> 0 holds
(fD (f,h)) . x = (2 * ((cos x) - (cos (x + h)))) / ((cos ((2 * x) + h)) + (cos h))