Si quieres calcularlo a pie:
1/h * (f(x+h)-f(x)) = 1/h * (1/(x+h) – 1/x)
= 1/h * (x-(x+h))/((x+h) * x)
= 1/h * (-h) / ((x+h) * x)
->-1/x^2 para h->0.