`A = 3x + 4y + 5`
`= -4y + 4y + 5`
`= 0 + 5`
`= 5`
`=> A = 5`
`B = 4x - 2y + 7`
`= 4x + 1/2. (-4y) + 7`
`= 4x + 1/2. 3x + 7`
`= 4x + 3/2x + 7`
`= 11/2x + 7`
Vì `A. B = (-5)/3`
`<=> 5. (11/2x + 7) = 5. (-1)/3`
`<=> 11/2x + 7 = -1/3`
`<=> 11/2x = -22/3`
`<=> x = -4/3`
`<=> -4y = 3x = 3. (-4)/3 = -4`
`<=> y = 1`
Vậy `x = -4/3; y = 1`