`D = x - x^2`
`D = -(x^2 - x)`
`D = -(x^2 - 2 . x . 1/2 + 1/4) + 1/4 `
`D = -(x - 1/2)^2 + 1/4 ≤ 1/4`
`text(Max)` `D = 1/4 khi x = 1/2`
`F = 2x + 4x^2 - 20`
`F = 4x^2 + 2 . 2x . 1/2 + 1/4 - 1/4 - 20`
`F = (2x + 1/2)^2 - 81/4 ≥ -81/4`
`text(Min)` `F = -81/4 khi x = -1/4`
`H = -3x(x + 3) - 7`
`H = -3x^2 - 9x - 7`
`H = -3(x^2 + 3x) - 7`
`H = -3(x^2 + 2 . x . 3/2 + 9/4) + 27/4 - 7`
`H = -3(x + 3/2)^2 - 1/4 ≤ -1/4`
`text(Max)` `H = -1/4 khi x = -3/2`