theorem :: DIFF_3:92

for h, x being Real
for f being Function of REAL,REAL st ( for x being Real holds f . x = (cot (#) sin) . x ) & x + (h / 2) in dom cot & x - (h / 2) in dom cot holds
(cD (f,h)) . x = (cos (x + (h / 2))) - (cos (x - (h / 2)))