1,`4x^2 - 4x + 3 = (2x-1)^2 + 2 `
vì `(2x-1)^2 >= ∀ x => (2x -1)^2 + 2>= 2 ∀ x `
vậy min = 2 khi `2x-1=0 => x =1/2`
2,`25x^2 + 10x + 1 = (5x+1)^2`
vì `(5x + 1)^2 >=0 ∀ x `
vậy min =0 khi ` 5x + 1 = 0 => x =-1/5`
3,`-x^2 + 8x + 5 = -(x^2 -8x + 16) + 21 = -(x-4)^2 + 21`
vì `-(x-4)^2 <=0 ∀ x => -(x-4)^2 + 21 <=21 ∀ x `
vậy max = 21 khi` x-4 = 0 => x=4`