https://elearn.univ-oran2.dz/pluginfile.php/365996/mod_resource/content/2/Cours%20L2%20seance1%20%D9%85%D9%82%D8%AF%D9%85%D8%A9%20%D8%B9%D8%A7%D9%85%D8%A9%20%D8%B9%D9%86%20%D8%A7%D9%84%D8%A8%D8%AD%D8%AB%20%D8%A7%D9%84%D8%B9%D9%84%D9%85%D9%8A.pdf