TeX source:

\begin{align} \begin{array}{l|l} \mathrm{x} & \mathrm{f}(\mathrm{x}) \\ \hline 1.1 & 3.2 \\ 1.003 & 3.006 \\ 1.0001 & 3.0002 \\ 1.000007 & 3.000014 \\ \quad \downarrow & \quad \downarrow \\ \quad \mathbf{1} & \quad \mathbf{3} \end{array} \end{align}