Bonsoir,
a/
x + y = 50
8x + 5y = 316
x = 50 - y
8 (50 - y) + 5y = 316
400 - 8y + 5y = 316
- 8y + 5y = 316 - 400
- 3y = - 84
y = 84/3
y = 28
x + y = 50
x + 28 = 50
x = 50 - 28
x = 22
Les solutions sont : (28 ; 22)
b/
x + y = 20
3x - 4y = 11
x = 20 - y
3 (20 - y) - 4y = 11
60 - 3y - 4y = 11
- 3y - 4y = 11 - 60
- 7y = - 49
y = 49/7
y = 7
x + y = 20
x + 7 = 20
x = 20 - 7
x = 13
Les solutions sont : (7 ; 13)
c/ Essaie le dernier