theorem :: DIFF_3:68

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 + (h / 2)) <> 0 & cos (x - (h / 2)) <> 0 holds
(cD (f,h)) . x = (2 * ((cos (x - (h / 2))) - (cos (x + (h / 2))))) / ((cos (2 * x)) + (cos h))