theorem :: DIFF_2:40

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