a)
{2 x - 3 y = 13
{4 x + y = 5
⇒ y = 5 - 4 x
_______________________
2 x - 3 ( 5 - 4 x ) = 13
2 x - 15 + 12 x = 13
14 x = 13 + 15
x = 28 : 14
x = 2
y = 5 - 4 × 2
y = 5 - 8
y = - 3
Verific: 2 × 2 - 3 × ( - 3) = 4 + 9 = 13
b)
{4 x - 3 y = 6
l ×5
{5 x - 7 y = 1
l ×4
___________________
20 x - 15 y = 30 → le scad
20 x - 28y = 4
_________________
/ 13 y = 26
y = 26 /13
y = 2
5 x - 7 y = 1
5 x - 7 × 2 = 1
5 x = 1 + 14
x = 15 : 5
x = 3
c)
{x + 3 y = - 1
{ 2 x - y = 5
___________
x = - 1 - 3 y
2 ( - 1 - 3 y ) - y = 5
- 2 - 6 y - y = 5
- 7 y = 5 + 2
y = 7 : ( - 7)
y = - 1
x = - 1 - 3 ( - 1) = - 1 + 3
x = 2
