If \(T_n(x)\) is the degree \(n\) Taylor approximation of \(f(x)\) at \(x=a\), then the approximation error \(R_n(x) = f(x) - T_n(x)\) (sometimes called remainder), in Lagrange mean-value form, is \[ R_n(x) = \frac{f^{(n+1)}(\xi_L)}{(n+1)!} (x-a)^{n+1}\] for some real number \(\xi_L\) between \(a\) and \(x\). (Technical hypotheses: this formula holds if \(f^{(n)}\) is continuous on the closed interval between \(a\) and \(x\), and differentiable on the open interval between \(a\) and \(x\).).