Développons : (x + 1)·(x -1) x·x + x·-1 + 1·x + 1·-1 ℹx·x = x2‖x·-1 = -x‖1·x = x‖1·-1 = -1 x2 -x + x -1 ℹ-x + x = 0 x2 + 0 -1 ▷Réponse : x2 -1