`a) P = x^2 - 5x +1`
`= x^2 - 2.5.1x + 25/4 - 21/4`
`= (x-5/2)^2 - 21/4 ≥ -21/4`
$\text{Vậy}$ `P_min = -21/4`
`⇔ x = 5/2`
`b) P = x^2 - 6x - 4`
`= x^2 - 2.3x + 9 - 13`
`= (x - 3)^2 - 13 ≥ -13 ∀ x ∈ RR`
$\text{Vậy}$ `P_min = -13`
`⇔ x = 3`
`c) P = 2x^2 - 4x + 6`
`= (x\sqrt[2])^2 - 2.\sqrt[2].\sqrt[2x] + 2 + 4`
`= (x\sqrt[2] - \sqrt[2])^2 + 4 ≥ 4 ∀ x ∈ RR`
$\text{Vậy}$ `P_min = 4`
`⇔ x = 1`
Xin hay nhất ạ mik rất cần nó thanks bn nhiều !