Réponse :
A = 3/7 + 4/21 * 5/2
= 3/7 + 10/21 (4/21 * 5/2 = 2/21 * 5= 10/21)
= 9/21 + 10/21 (3/7 = (3*3)/(7*3) = 9/21)
= 19/21
______
B = (3/7 - 4/21) * 5/2
= (9/21 - 4/21) * 5/2
= (5/21) * 5/2
= 25/42
______
C = 2 + 1/3 * 3/5 + 2
= 2 + 2 + 1/5 (1/3 * 3/5 = 1 * 1/5 = 1/5)
= 4 + 1/5
= 20/5 + 1/5
= 21/5
Voilà
