Répondre :
-5/15 + -26/18 + -35/-21 = -5/15 -26/18 +35/21 = -5x1/5x3 - 26/18 + 7x5/7x3
= -1/3 - 26/18 + 5/3
= 4/3 - 26/18
= 4x6/3x6 - 26/18
= 24/18 -26/18 = -2/18= -1/9
-7sur63 car
-5 sur15 + -6sur18 = -16sur9 et -16sur9+35sur21 car on a enlevé le - pour les deux = -7sur63