Source: http://fsjes-agdal.um5.ac.ma/sites/fsjes-agdal.um5.ac.ma/files/Exercices%20et%20examens%20corrig%C3%A9s%20Micro%C3%A9conomie%20I%20S1%20%28Pr%20ZOUIRI%20Lahboub%29.pdf