1 = -1

Dans le même genre que 1 + 1 = 1:

1: -a/a = a/-a
2: (-a/a)^1/2 = (a/-a)^1/2
3: (-a)^(1/2) / a^(1/2) = a^(1/2) / (-a)^(1/2)
4: (-a)^(1/2) * (-a)^(1/2) = a^(1/2) * a^(1/2)
5: ((-a)^(1/2))² = (a^(1/2))²
6: (-a)^(2/2) = a^(2/2)
7: -a = a
8: -1 = 1