Answer
`D = 3x^2 + x - 1`
`D = 3 . (x^2 + 1/3x - 1/3)`
`D = 3 . [x^2 + 2 . x . 1/6 - (1/6)^2 - 35/36]`
`D = 3 . [(x - 1/6)^2 - 35/36]`
Nhận xét:
`(x - 1/6)^2 >= 0 AA x`
`=> (x - 1/6)^2 - 35/36 >= -35/36 AA x`
`=> 3 . [(x - 1/6)^2 - 35/36] >= {-35}/12 AA x`
`=> D >= {-35}/12`
Dấu $"="$ xảy ra:
`<=> x - 1/6 = 0`
`<=> x = 1/6`
Vậy `D_{min} = {-35}/12` khi `x = 1/6.`
