Bonjour :
1)
A = 15×(10²)⁵×0.3×10⁻³/9×10⁴
A = 15×0.3×10²ˣ⁵×10⁻³/9×10⁴
A = 4.5×10¹⁰×10⁻³/9×10⁴
A = 4.5×10¹⁰⁺⁽⁻³⁾/9×10⁴
A = 4.5×10¹⁰⁻³/9×10⁴
A = 4.5×10⁷/9×10⁴
A = 4.5/9 × 10⁷/10⁴
A = 0.5×10⁷⁻⁴
A = 0.5×10³
A = 5×10⁻¹×10³
A = 5×10⁻¹⁺³
A = 5×10² -----> L'écriture scientifique
2)
B = 1+2/3/3/10
B = 1+2/3×10/3
B = 1+2/10
B = 1+1/5
B = 5/5+1/5
B = 6/5
3)
C = 7/9-5/9÷5/3
C = 7/9-5/9×3/5
C = 7/9 - 3/9
C = 4/3 ---> 1.333
J’espère t'avoir t'aider