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\newcommand{\titre}{La tournée du facteur}
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    \small Un facteur veut parcourir toutes les rues de son quartier en n'empruntant chacune qu'une seule fois. Selon le plan, c'est tantôt facile, tantôt impossible — et l'on peut le dire d'avance, sans essayer, en regardant simplement combien de rues aboutissent à chaque carrefour.
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\subsection*{\textcolor{red}{La tournée du facteur}}
\noindent Voici le plan d'un quartier.\par\addvspace{0.35em}
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\end{center}
\noindent \textbf{Le facteur part de la poste et doit emprunter chaque rue exactement une fois. Il n'est pas obligé d'y revenir : il termine sa tournée où il veut.}\par\addvspace{0.35em}
\begin{center}
\renewcommand{\arraystretch}{1.35}
\begin{tabular}{|c|c|c|c|}
\hline
\cellcolor{blue!7}\bfseries Les rues & \cellcolor{blue!7}\bfseries  & \cellcolor{blue!7}\bfseries Les carrefours & \cellcolor{blue!7}\bfseries Les bâtiments \\ \hline
① rue Verte & ⑪ allée des Tilleuls & S0 Place du Marché & \textbf{a} la poste \\ \hline
② rue des Écoles & ⑫ chemin des Vignes & S1 Rond-point de la Poste & \textbf{b} l'église \\ \hline
③ chemin de Ronde & ⑬ rue Basse & S2 Coin du Lavoir & \textbf{c} la mairie \\ \hline
④ chemin Creux & ⑭ venelle du Chat & S3 Carrefour de l'Hôpital & \textbf{d} le gymnase \\ \hline
⑤ rue du Four & ⑮ sentier des Prés & S4 Place Sainte-Croix & \textbf{e} la boulangerie \\ \hline
⑥ rue Haute & ⑯ rue de la Mairie & S5 Rond-point du Pont &  \\ \hline
⑦ rue du Moulin & ⑰ rue des Lilas & S6 Carrefour du Moulin &  \\ \hline
⑧ rue Pasteur & ⑱ allée des Peupliers & S7 Rond-point de la Gare &  \\ \hline
⑨ rue Neuve & ⑲ rue des Jardins & S8 Carrefour Saint-Roch &  \\ \hline
⑩ boulevard du Parc & ⑳ rue du Pont & S9 Place du Vieux-Puits &  \\ \hline
 &  & S10 Place des Tanneurs &  \\ \hline
 &  & S11 Carrefour du Lavoir &  \\ \hline
\end{tabular}
\end{center}
\noindent{\small\itshape Légende du plan}\par\addvspace{0.4em}
\noindent Les questions se suivent : la première n'est qu'un comptage, la dernière demande une preuve.\par\addvspace{0.45em}
\begin{enumerate}
  \item Pour \textbf{chaque carrefour}, compter le nombre de rues qui y aboutissent. Présenter les résultats dans un tableau.
  \item Le facteur imagine sa tournée. À chaque fois qu'il \textbf{traverse} un carrefour, combien de rues utilise-t-il ? Qu'en déduire pour les carrefours où aboutit un nombre \textbf{impair} de rues ?
  \item \textbf{Le facteur peut-il faire sa tournée en partant de la poste ?} Si oui, décrire un trajet possible ; sinon, démontrer que c'est impossible.
  \item Si ce n'est pas possible : une tournée existe-t-elle \textbf{malgré tout} dans ce quartier, en partant d'ailleurs ? Si oui, où faudrait-il installer la poste — et le facteur y reviendrait-il ?
\end{enumerate}
\newpage
\section*{\textcolor{red}{Correction}}
\needspace{5\baselineskip}
\subsection*{\textcolor{red}{La tournée du facteur — Correction}}
\needspace{3\baselineskip}
\begin{center}
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\noindent \textbf{1.} On compte les rues aboutissant à chaque carrefour :\par\addvspace{0.35em}
\begin{center}
\renewcommand{\arraystretch}{1.35}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|c|c|c|}
\hline
\cellcolor{blue!7}\bfseries Carrefour & \cellcolor{blue!7}\bfseries S0 & \cellcolor{blue!7}\bfseries S1 & \cellcolor{blue!7}\bfseries S2 & \cellcolor{blue!7}\bfseries S3 & \cellcolor{blue!7}\bfseries S4 & \cellcolor{blue!7}\bfseries S5 & \cellcolor{blue!7}\bfseries S6 & \cellcolor{blue!7}\bfseries S7 & \cellcolor{blue!7}\bfseries S8 & \cellcolor{blue!7}\bfseries S9 & \cellcolor{blue!7}\bfseries S10 & \cellcolor{blue!7}\bfseries S11 \\ \hline
Nombre de rues & 2 & 4 & 2 & 6 & 2 & 4 & 4 & 3 & 5 & 2 & 2 & 4 \\ \hline
Parité & pair & pair & pair & pair & pair & pair & pair & \textbf{impair} & \textbf{impair} & pair & pair & pair \\ \hline
\end{tabular}
\end{center}
\noindent\hspace{1.2em}Vérification : la somme de ces nombres vaut 40 = 2 \(\times\) 20, car chaque rue est comptée à ses \textbf{deux} extrémités.\par\addvspace{0.3em}
\noindent{\color{red!80!black}\bfseries Les carrefours à nombre \textbf{impair} de rues sont : \textbf{S7}, \textbf{S8} — il y en a 2.}\par\addvspace{0.4em}
\noindent \textbf{2.}\par\addvspace{0.35em}
\noindent{\color{green!45!black}\textbf{Le raisonnement clé.} Quand le facteur \textbf{traverse} un carrefour, il y arrive par une rue et en repart par une autre : il consomme les rues \textbf{deux par deux}. Un carrefour traversé 3 fois voit donc 6 de ses rues utilisées — toujours un nombre \textbf{pair}.}\par\addvspace{0.4em}
\noindent{\color{red!80!black}\bfseries Un carrefour où aboutit un nombre \textbf{impair} de rues ne peut donc pas être seulement traversé : il doit être le \textbf{départ} ou l'\textbf{arrivée} de la tournée. Là, une rue reste « non appariée ».}\par\addvspace{0.4em}
\noindent{\color{green!45!black}Ici la poste est \textbf{sur un carrefour}, en \textbf{S7}, et le facteur n'a pas à y revenir. Ce carrefour est donc le \textbf{départ} — il a droit d'être impair. Mais il n'y a qu'\textbf{une seule arrivée} : tous les autres carrefours doivent être pairs, sauf un au plus.}\par\addvspace{0.4em}
\noindent \textbf{3.}\par\addvspace{0.35em}
\noindent{\color{red!80!black}\bfseries \textbf{Oui.} Deux carrefours sont impairs, \textbf{S7} et \textbf{S8}, et la poste est justement sur l'un des deux (\textbf{S7}). Il part donc de la poste et \textbf{termine en S8}, l'autre carrefour impair.}\par\addvspace{0.4em}
\newpage
\noindent Voici une tournée possible :\par\addvspace{0.35em}
\begin{enumerate}
  \item Il part de Rond-point de la Gare \textbf{(S7)}, prend le \textbf{boulevard du Parc ⑩} vers le nord et atteint Carrefour de l'Hôpital \textbf{(S3)}.
  \item Il continue tout droit dans la \textbf{rue des Écoles ②}, vers le nord, jusqu'à Place du Marché \textbf{(S0)}.
  \item Il tourne à droite dans la \textbf{rue Verte ①}, vers l'est, jusqu'à Rond-point de la Poste \textbf{(S1)}.
  \item Il tourne à droite dans le \textbf{chemin de Ronde ③}, vers le sud-ouest, jusqu'à Carrefour de l'Hôpital \textbf{(S3)}.
  \item Il tourne à droite dans la \textbf{rue du Moulin ⑦}, vers le nord-ouest, jusqu'à Coin du Lavoir \textbf{(S2)}.
  \item Il prend dans la \textbf{rue Pasteur ⑧}, vers le sud, jusqu'à Carrefour Saint-Roch \textbf{(S8)}.
  \item Il tourne à gauche dans l'\textbf{allée des Tilleuls ⑪}, vers le nord-est, jusqu'à Carrefour de l'Hôpital \textbf{(S3)}.
  \item Il tourne à droite dans la \textbf{rue Neuve ⑨}, vers l'est, jusqu'à Rond-point du Pont \textbf{(S5)}.
  \item Il tourne à gauche dans la \textbf{rue du Four ⑤}, vers le nord, jusqu'à Rond-point de la Poste \textbf{(S1)}.
  \item Il tourne à droite dans le \textbf{chemin Creux ④}, vers le sud-est, jusqu'à Place Sainte-Croix \textbf{(S4)}.
  \item Il tourne à droite dans le \textbf{chemin des Vignes ⑫}, vers l'ouest, jusqu'à Rond-point du Pont \textbf{(S5)}.
  \item Il tourne à gauche dans la \textbf{rue Basse ⑬}, vers le sud, jusqu'à Carrefour du Moulin \textbf{(S6)}.
  \item Il tourne à droite dans la \textbf{venelle du Chat ⑭}, vers le sud-ouest, jusqu'à Carrefour du Lavoir \textbf{(S11)}.
  \item Il tourne à droite dans la \textbf{rue Haute ⑥}, vers l'ouest, jusqu'à Place des Tanneurs \textbf{(S10)}.
  \item Il tourne à droite dans l'\textbf{allée des Peupliers ⑱}, vers le nord, jusqu'à Carrefour Saint-Roch \textbf{(S8)}.
  \item Il tourne à droite dans la \textbf{rue des Lilas ⑰}, vers l'est, jusqu'à Rond-point de la Gare \textbf{(S7)}.
  \item Il continue tout droit dans le \textbf{sentier des Prés ⑮}, vers l'est, jusqu'à Carrefour du Moulin \textbf{(S6)}.
  \item Il tourne à droite dans la \textbf{rue de la Mairie ⑯}, vers le sud, jusqu'à Place du Vieux-Puits \textbf{(S9)}.
  \item Il tourne à droite dans la \textbf{rue du Pont ⑳}, vers l'ouest, jusqu'à Carrefour du Lavoir \textbf{(S11)}.
  \item Il continue tout droit dans la \textbf{rue des Jardins ⑲}, vers le nord-ouest, jusqu'à Carrefour Saint-Roch \textbf{(S8)}.
\end{enumerate}
\noindent \textbf{4.}\par\addvspace{0.35em}
\noindent{\color{red!80!black}\bfseries C'était déjà possible. Notons seulement que la poste devait être en \textbf{S7} ou en \textbf{S8} : ailleurs, la tournée n'aurait pas pu commencer.}\par\addvspace{0.4em}
\end{document}
