`text{a) -9x² + 24 - 18}`
`text{= -(9x² + 24 - 18}`
`text{= -(9x² - 24 - 16 - 2)}`
`text{= -(9x² - 24 - 16) - 2}`
`text{= -(3x - 4)² - 2}`
`text{Vì (3x - 4)² ≥ 0}` `AA` `text{x}`
`text{⇒ -(3x - 4)² ≤ 0}` `AA` `text{x}`
`text{⇒ -(3x - 4)² - 2 ≤ -2}`
`text{⇔ Max A = -2 }`
`text{Dấu "=" xảy ra ⇔ (3x - 4) = 0}`
`text{ ⇔ 3x - 4 = 0}`
`text{ ⇔ x =}` `4/3`
`text{Vậy Max A = -2 ⇔ x =}` `4/3`
`text{___________________________________________________________}`
`text{b) 4x - x² - 1}`
`text{= -(4x - x² + 1)}`
`text{= -(x² - 4x + 4 - 3)}`
`text{= -(x - 2)² - 3}`
`text{Vì: (x - 2)² ≥ 0}` `AA` `text{x}`
`text{⇒ -(x - 2)² ≤ 0}` `AA` `text{x}`
`text{⇒ -(x - 2)² + 3 ≤ 3}`
`text{⇔ Max B = 3}`
`text{Dấu "=" xảy ra ⇔ (x - 2) = 0}`
`text{ ⇔ x - 2 = 0}`
`text{ ⇔ x = 2}`
`text{Vậy Max B = 3 ⇔ x = 2}`
