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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 chemin des Vignes & ⑩ chemin du Lavoir & Ⓐ Place Sainte-Croix & \textbf{a} la poste \\ \hline
② quai aux Fleurs & ⑪ rue du Moulin & Ⓑ Rond-point de la Gare & \textbf{b} le stade \\ \hline
③ rue du Four & ⑫ rue Haute & Ⓒ Carrefour Saint-Roch & \textbf{c} la bibliothèque \\ \hline
④ rue des Écoles & ⑬ rue Neuve & Ⓓ Carrefour du Lavoir & \textbf{d} l'école \\ \hline
⑤ avenue de la Gare & ⑭ chemin Creux & Ⓔ Carrefour de l'Hôpital & \textbf{e} l'église \\ \hline
⑥ rue de l'Église & ⑮ allée des Peupliers & Ⓕ Carrefour des Tilleuls &  \\ \hline
⑦ allée des Tilleuls & ⑯ rue du Puits & Ⓖ Rond-point des Vaches &  \\ \hline
⑧ rue des Lilas & ⑰ rue des Jardins & Ⓗ Carrefour des Quatre-Chemins &  \\ \hline
⑨ chemin de Ronde &  & Ⓘ Fontaine Notre-Dame &  \\ \hline
 &  & Ⓙ Place du Marché &  \\ \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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\end{center}
\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|}
\hline
\cellcolor{blue!7}\bfseries Carrefour & \cellcolor{blue!7}\bfseries A & \cellcolor{blue!7}\bfseries B & \cellcolor{blue!7}\bfseries C & \cellcolor{blue!7}\bfseries D & \cellcolor{blue!7}\bfseries E & \cellcolor{blue!7}\bfseries F & \cellcolor{blue!7}\bfseries G & \cellcolor{blue!7}\bfseries H & \cellcolor{blue!7}\bfseries I & \cellcolor{blue!7}\bfseries J \\ \hline
Nombre de rues & 2 & 3 & 4 & 3 & 4 & 4 & 4 & 6 & 2 & 2 \\ \hline
Parité & pair & \textbf{impair} & pair & \textbf{impair} & pair & pair & pair & pair & pair & pair \\ \hline
\end{tabular}
\end{center}
\noindent\hspace{1.2em}Vérification : la somme de ces nombres vaut 34 = 2 \(\times\) 17, 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{B}, \textbf{D} — 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{D}, 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{B} et \textbf{D}, et la poste est justement sur l'un des deux (\textbf{D}). Il part donc de la poste et \textbf{termine en B}, l'autre carrefour impair.}\par\addvspace{0.4em}
\newpage
\noindent Voici une tournée possible :\par\addvspace{0.35em}
\begin{enumerate}
  \item Il part de Carrefour du Lavoir \textbf{(D)}, prend la \textbf{rue de l'Église ⑥} vers l'ouest et atteint Carrefour Saint-Roch \textbf{(C)}.
  \item Il prend dans le \textbf{chemin des Vignes ①}, vers l'est, jusqu'à Place Sainte-Croix \textbf{(A)}.
  \item Il tourne à droite dans le \textbf{quai aux Fleurs ②}, vers le sud, jusqu'à Carrefour des Tilleuls \textbf{(F)}.
  \item Il tourne à droite dans la \textbf{rue des Lilas ⑧}, vers le nord-ouest, jusqu'à Carrefour du Lavoir \textbf{(D)}.
  \item Il tourne à gauche dans le \textbf{chemin de Ronde ⑨}, vers le sud-ouest, jusqu'à Carrefour des Quatre-Chemins \textbf{(H)}.
  \item Il tourne à droite dans la \textbf{rue du Moulin ⑪}, vers le nord-ouest, jusqu'à Carrefour de l'Hôpital \textbf{(E)}.
  \item Il continue tout droit dans la \textbf{rue des Écoles ④}, vers le nord-ouest, jusqu'à Rond-point de la Gare \textbf{(B)}.
  \item Il prend dans la \textbf{rue du Four ③}, vers l'est, jusqu'à Carrefour Saint-Roch \textbf{(C)}.
  \item Il prend dans l'\textbf{allée des Tilleuls ⑦}, vers le sud-ouest, jusqu'à Carrefour de l'Hôpital \textbf{(E)}.
  \item Il tourne à droite dans le \textbf{chemin du Lavoir ⑩}, vers l'ouest, jusqu'à Rond-point des Vaches \textbf{(G)}.
  \item Il prend dans le \textbf{chemin Creux ⑭}, vers l'est, jusqu'à Carrefour des Quatre-Chemins \textbf{(H)}.
  \item Il tourne à gauche dans la \textbf{rue Haute ⑫}, vers l'est, jusqu'à Carrefour des Tilleuls \textbf{(F)}.
  \item Il tourne à droite dans la \textbf{rue Neuve ⑬}, vers le sud, jusqu'à Fontaine Notre-Dame \textbf{(I)}.
  \item Il tourne à droite dans la \textbf{rue du Puits ⑯}, vers le nord-ouest, jusqu'à Carrefour des Quatre-Chemins \textbf{(H)}.
  \item Il tourne à gauche dans la \textbf{rue des Jardins ⑰}, vers le sud, jusqu'à Place du Marché \textbf{(J)}.
  \item Il tourne à droite dans l'\textbf{allée des Peupliers ⑮}, vers le nord-ouest, jusqu'à Rond-point des Vaches \textbf{(G)}.
  \item Il tourne à droite dans l'\textbf{avenue de la Gare ⑤}, vers le nord, jusqu'à Rond-point de la Gare \textbf{(B)}.
\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{B} ou en \textbf{D} : ailleurs, la tournée n'aurait pas pu commencer.}\par\addvspace{0.4em}
\end{document}
