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\begin{document}
\begin{center}
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\begin{tabular}{|*3{c|}>{\centering\arraybackslash}p{3cm}|}
\hline
Tailles (en cm) & Effectifs $n_i$ & Centre des classes $x_i$ & $n_ix_i^2$\\\hline\hline
[160;170[ &35 & & \\\hline
[170;180[ &72 &&\\\hline
[180;190[ &47 &&\\\hline
[190;200[ &28 &&\\\hline
[200;210] &18 &&\\\hline\hline
Totaux : & & \diagbox[width=41mm,height=8mm]{\diagbox[width=41mm,height=8mm,dir=SW]{}{}}{} &  \\\hline
\end{tabular}
\end{center}
\end{document}