Bonsoir, je t'aide que pour l'exercice 1 :

a- 2 x 10^3 x 5 x 10^2
= 10 x 10^3+2
= 10 x 10^5 = 10^6

b- 7 x 10^5 x 11 x 10-2
= 77 x 10^5+(-2)
= 77 x 10^3
= 7.7 x 10^4

c- 5 x (10^-6)^2 x 3 x 10^-6
= 15 x 10^-6x2 x 10^-6
= 15 x 10^-12+(-6)
= 15 x 10^-18
= 1.5 x 10^-17