Calculer les longueurs $HE$ et $HD$ \begin{center} \includegraphics{./fig/situation_thales} \end{center} .\dotfill \\[0.3cm] .\dotfill \\[0.3cm] .\dotfill \\[0.3cm] .\dotfill \\[0.3cm] .\dotfill \\[0.3cm] .\dotfill \\[0.3cm] .\dotfill \\[0.3cm] .\dotfill \\[0.3cm] .\dotfill \\[0.3cm]