Exercice 3 et 4 svpppp????? :((

4/21 + 5/3 = 4/21 + 5 X 7 / 3 X 7 = 4/21 + 35/21 = 39/21

4/9 - 7/15 = 4 X 5 / 9 X 5 - 7 X 3 / 15 X 3 = 20/45 - 21/45 = - 1/45

Voilà les 2 premiers