theorem :: DIFF_3:52

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