`A=(x-2)^2+(2x-1)^2`
`A=x^2 - 4x + 4 + 4x^2 - 4x + 1`
`A=5x^2-8x`
`A=5(x^2 - 8/5 x + 1 )`
`A=5(x^2- 2 . x . 4/5 + 16/25 - 16/25 + 1 )`
`A=5[(x-4/5)^2+9/5` $\geq$ `9/5`
VÌ `5(x-4/5)^2` $\geq$ 0
Dấu ''='' xảy ra
`5(x-4/5)^2=0 `
⇒ `x = 4/5`
Vậy `Amin = 9/5 ⇔ x=4/5`
