\begin{exercice}[Calcul mental]
\vspace{-1.5em}
\begin{enumerate}
\item Factoriser $(x+1)^2-(x-1)^2$.
\item Calculer $10001^2-9999^2$.
%\item Calcule $123456788 \times 123456790 - 123456788$
\end{enumerate}
\end{exercice}