theorem Th34: :: DIFF_2:34

for x0, x1, k being Real
for f being Function of REAL,REAL st ( for x being Real holds f . x = k / x ) & x0 <> x1 & x0 <> 0 & x1 <> 0 holds
[!f,x0,x1!] = - (k / (x0 * x1))