theorem :: DIFF_3:84

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