il teorema fondamentale del calcolo:   f(b) - f(a) = ab f'(x) dx .
  f(b) - f(a) = Σk=0..n-1 { [ f(xk+Δx)-f(xk) ]/Δx } Δx = Σk=0..n-1 f'(ξk)Δx , con xk < ξk < xk+1

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