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\begin{exercise}
Soit une s\'{e}rie \`{a} deux caract\`{e}res $x_{1}$ et $x_{2}$ telle que :
\[
x_{1}-\overline{x}=-1\quad\text{et}\quad x_{2}-\overline{x}=2
\]
Sachant que l'effectif $n_{1}$ de $x_{1}$ est 4 et que l'\'{e}cart type
$\sigma$ de cette s\'{e}rie est $\sqrt{2}$, d\'{e}terminer l'effectif $n_{2}$
de $x_{2}$.
\end{exercise}


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