Bonjour !
Rendre au même denominateur : 4
A = 5/4 - 3/2
→ 5/4 = 5/4
→ 3/2 = 3×2/2×2 = 6/4
=> A = 5/4 - 6/4 = -1/4
Dénominateur commun : 24
B = 7/8 + 5/3
→ 7/8 = 7×3/8×3 = 21/24
→ 5/3 = 5×8 /3×8 = 40/24
=> B = 21/24 +40/24 = 61/24
Dénominateur commun : 12
C = -1+5/4-7/6
→ -1 = 1/1 = 1×12/1×12 = -12/12
→ 5/4 = 5×3 / 4×3 = 15/12
→ 7/6 = 7×2 /6×2 = 14/12
=> C = -12/12 + 15/12 - 14/12 = -11/12
Bon courage.
