Bonjour Help me please c'est pour la rentrée merci d'avance.

bonjour

exercice 1

a = 2 - 2 x 8/5
a = 2 - 16/5
a = 10/5 - 16/5
a = - 6/5

b = ( 2 - 4/5 + 1/3) : ( 2 + 4/5 + 1/3)
b = ( 30/15 - 12/15 + 5/15) : ( 30/15 + 12/15 + 5/15)
b = 23/15 : 47/15
b = 23/15 x 15/47 = 345/705 = 23/47

exercice 2

a = 8 x 10⁻⁴⁰ x 12 x 10 ²¹ sur 0.5 x 10³ x 4 x 10 ⁻¹¹
a = 96 x 10 ⁻¹⁹ sur 2 x 10 ⁻⁸
a = 48 x 10 ⁻¹¹
a = 4.8 x 10⁻¹⁰

b = 30 x 10  sur 15 x 10³
b = 2 x 10 ⁻²

c = 1.47 x 10 ¹⁷ sur 63 x 10 ¹⁵
c = 0.023 x 10 ² = 2.3 x 10 ⁰

exercice 3 

AD/AB = AE/AC = DE/BC

ad/7.5 = 4/6
6 ad = 7.5 x 4
6 ad = 30
ad = 5