3) `{(3x - 5y = -18),(x + 2y = 5):}`

`<=> {(3x - 5y = -18),(3x + 6y = 15):}`

`<=> {(-11y = -33),(3x + 6y = 15):}`

`<=> {(y = 3),(3x + 6*3 = 15):}`

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

Vậy `(x;y) = (-1;3)`

4) `{(4x + 3y = -6),(2x - 5y = 16):}`

`<=> {(4x + 3y = -6),(4x - 10y = 32):}`

`<=> {(13y = - 38),(4x - 10y = 32):}`

`<=> {(y = - 38/13),(4x - 10 * (-38/13) = 32):}`

`<=> {(y = - 38/13),(x = 9/13):}`

Vậy `(x;y) = (9/13; - 38/13)`

5) `{(2x - y = x + 3y + 3),(3x - 3y = 9):}`

`<=> {(2x - y - x - 3y = 3),(3(x-y) = 9):}`

`<=> {(x - 4y = 3),(x - y = 3):}`

`<=> {(-3y = 0),(x - y = 3):}`

`<=> {(y = 0),(x - 0 = 3):}`

`<=> {(y = 0),(x = 3):}`

Vậy `(x;y) = (3;0)`

`\ttcolor{cyan}{#PUyen}`