Réponses

2013-05-19T17:59:49+02:00

(a²-b²-ab2)+(ab-b²)+(ab-a²)= a² -b² -2ab +ab - b² +ab -a²

                                           = a² - a² -b² -b² -2ab  +ab +ab

                                           = -2b²

2013-05-19T18:47:36+02:00

a²-b²-2ab+ab-b²+ab-a² = -2b²  (tout s'annule sauf ca!)