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\begin{document}
Voici quelques exemples :
\begin{align}
   f(x) & = x^2 + 8x + 16 \\
   & = (x+4)^2
\end{align}

\begin{eqnarray}
   x + 3y - 6z & = & -4a + 5b -7b \\
               & = & -4a - 2b
\end{eqnarray}

\begin{align}
   \begin{split}
      A &= B + C + D + E + F + G \\
      &\quad + H + I
   \end{split} \\
   F &= G + H
\end{align}
\end{document}
