This means that the global error goes like $\approx O(h^2)$. The trapezoidal reads then $$\begin{equation} I=\int_a^bf(x) dx=h\left(f(a)/2 + f(a+h) +f(a+2h)+ \dots +f(b-h)+ f_{b}/2\right), \tag{4} \end{equation}$$ with a global error which goes like $O(h^2)$.

Hereafter we use the shorthand notations $f_{-h}=f(x_0-h)$, $f_{0}=f(x_0)$ and $f_{h}=f(x_0+h)$.