theorem :: DIFF_3:76

for h, x being Real
for f being Function of REAL,REAL st ( for x being Real holds f . x = 1 / ((cos x) ^2) ) & cos (x + (h / 2)) <> 0 & cos (x - (h / 2)) <> 0 holds
(cD (f,h)) . x = (((((- 16) * (sin x)) * (sin ((- h) / 2))) * (cos x)) * (cos ((- h) / 2))) / (((cos (2 * x)) + (cos h)) ^2)