developpe (x+1)*(x-1)
Développons : (x + 1)·(x -1)
x·x + x·-1 + 1·x + 1·-1
x·x = x2x·-1 = -x1·x = x1·-1 = -1
x2 -x + x -1
-x + x = 0
x2 + 0 -1
Réponse :   x2 -1