Réponse:

a)

-2+1 ≤ x+y ≤ 6+4

-1 ≤ x+y ≤ 10

b)

-2 ≤ x ≤ 6 et 1≤y≤4

-4 ≤ 2x ≤ 12 et 3 ≤ 3y ≤ 12

-4+3 ≤ 2x+3y ≤ 12+12

-1 ≤ 2x+3y ≤ 24

c)

1≤y≤4

-1×1 ≥ -1×y ≥ -1×4

-4 ≤ -y ≤ -1

d)

-2 ≤ x ≤ 6 et -4 ≤ -y ≤ -1

-2 +(-4) ≤ x+(-y) ≤ 6+(-1)

-6 ≤ x-y ≤ 5