% Fichier exo4.13.tex - Ch. Rolland/O Reilly France
\documentclass{article}
\usepackage[latin1]{inputenc}
\begin{document}
$$
\left\{
\begin{array}{l@{\quad}l}
y_i=ax_i+b & si\ i \not= 0\\
y_i=1      & si\ i = 0    \\
\end{array}
\right.
$$
\end{document}
