Explications étape par étape:
1)
-1<= X <=1 , 0<= Y <=1/2
-1+0<= X+Y <= 1+1/2
-1<= X+Y <= 2/2+ 1/2
-1<= X+Y <= 3/2
(1/2) positive donc
(1/2) ×-1 <= (1/2)X+Y <= (1/2) × 3/2
-1/2 <= X+Y/2 <= 3/4
2) on a 0<= Y <=1/2
(-1) est négative donc
-1/2 <= -Y <= 0
-1/2 <= -Y <= 0. +. -1<= X <=1
-1/2+( -1) <= X-Y <= 1+0
-3/2 <= X-Y <= 1
