`A=x^2-6x+1`
`A=x^2-6x+9-8`
`A=(x-3)^2-8`
Ta có: `(x-3)^2≥0`
`⇒(x-3)^2-8≥-8`
Vậy Min `A=-8` đạt khi `x=3`
`B=2x^2+10x-1`
`B=x^2+5x+x^2+5x-1`
`B=x^2+5x+25/4+x^2+5x+25/4-27/2`
`B=(x+5/2)^2+(x+5/2)^2-27/2`
Ta có: `(x+5/2)^2≥0`
`⇒(x+5/2)^2+(x+5/2)^2-27/2≥-27/2`
Vậy Min `B=-27/2` đạt khi `x=-5/2`
`C=5x-x^2`
`C=-(x^2-5x)`
`C=-(x^2-5x+25/4-25/4)`
`C=-(x-5/2)^2+25/4 `
Ta có: `(x-5/2)^2≥0`
`⇒-(x-5/2)^2≤0`
`⇒-(x-5/2)^2+25/4≤25/4`
Vậy Max `C=25/4` đạt khi `x=5/2`
