5x-4y = 23
3x+8y = 7
donc 5x = 23 +4y x = 23/5 +4/5y
donc
3x+8y = 7
3 (23/5 +4/5y) +8y =7
69/5 +12/5y +8y =7
12/5y +40/5 y = 7 -69/5
52/5y = 35/5 -69/5
52/5y = -34/5
y = -34/5 * 5/52
y = -34/52
y = -17/26
x =23/5+4/5y = -23/5 +4/5(-17/26) = -23/5 -34/65 = -169/65 -34/65 = -203/65
b/
2x +5y = -4
x-2y = 7
x = 7 +2y
2x+5y = -4
2(7+2y) +5y =-4
14 +4y +5y = -4
9y = -4 -14
9y = -28
y = -28/9
x = 7+2y
x =7 +2* -28/9
x = 7 -56/9
x =63/9-56/9
x = 7/9