Source: http://perso.eleves.ens-rennes.fr/people/clementine.laurens/Documents%20page%20web%20perso/CPGE/Cours%20et%20fiches%20de%20r%C3%A9vision/Sommes%20-%20Formules,%20m%C3%A9thodes%20de%20calcul%20et%20sommes%20classiques.pdf