theorem :: DIFF_3:50

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 <> 0 & x + h <> 0 holds
(fD (f,h)) . x = (((- k) * h) * ((2 * x) + h)) / (((x ^2) + (h * x)) ^2)