theorem :: DIFF_3:63

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