4) 2x = 3
x = 3/2
5) -5x = -25/8
x = -25/8 / (-5)
x = -25/8 * (-1/5)
x = 25/40
x = (5*5) / (5*8)
x = 5/8
6 ) x / (-5) / 7/5 produit en croix
5x = (-5) * 7
5x = -35
x = -35 / 5
x = -7
2) On remplace x par -4:
4x = 4 * (-4) = -16 ≠ 0
La solution -4 est donc fausse