Bonjour !
A = (2/5-5/12) × (-5/3)
A = (2/5-5/12) = 24-25/60 = - 1/60
A = - 1/60 × - 5/3 = 5/180 = 1/36
B = 4/3-2/7 : (6/21)²
Numérateur :
4/3-2/7 = 28-6 : 21 = 22/21
Dénominateur :
(6/21)² = 36/441
B = 22/21 : 36/441
B = 22/21 × 441/36 = 77/6
C =23,5 ×10^7×2×10^5
C = 2,35×10^8×2×10^5
C = 18,8x10^13
C = 1,88× 10^14
Bonne journée.
