a) `{(2x + y = 4),(x - y = 2):}`

`<=> {(3x = 6),(x - y = 2):}`

`<=> {(x = 2),(2 - y =2):}`

`<=> {(x = 2),(y = 0):}`

Vậy `(x;y) = (2;0)`

b) `{(4x + 5y = 11),(2x - 3y = 0):}`

`<=> {(4x + 5y = 11),(4x - 6y = 0):}`

`<=> {(11y = 11),(4x - 6y = 0):}`

`<=> {(y = 1),(4x - 6*1 = 0):}`

`<=> {(y = 1),(x = 3/2):}`

Vậy `(x;y) = (3/2; 1)`

c) `{(12x + 18y = -24),(-2x - 3y = 4):}`

`<=> {(12x + 18y = -24),(-12x - 18y = 24):}`

`<=> {(0x + 0y = 0  (luôn  đúng)),(-12x - 18y = 24):}`

Vậy hpt có vô số nghiệm

d) `{(x - 3y = 5),(-2x + 6y = 10):}`

`<=> {(2x - 6y = 10),(-2x + 6y = 10):}`

`<=> {(0x + 0y = 20  (vô  lý)),(-2x + 6y = 10):}`

Vậy hpt vô nghiệm

`\ttcolor{cyan}{#PUyen}`