`@` `Q`

Đáp án:

`A=x^2+2x+2`

`A=x^2+2x+1+1`

`A=(x+1)^2+1`

Ta có :

`(x+1)^2 ge 0 AA x`

`to A=(x+1)^2+1 ge 1 AA x`

Dấu "=" xảy ra khi : `(x+1)^2=0 to x+1=0 to x=-1`

Vậy `A_(min)=1⇔x=-1`

`*********`

`B=x^2-x+3`

`B=x^2-x+1/4+11/4`

`B=(x-1/2)^2+11/4`

Ta có :

`(x-1/2)^2 ge 0 AA x`

`to B=(x-1/2)^2+11/4 ge 11/4 AA x`

Dấu "=" xảy ra khi : `(x-1/2)^2=0 to x-1/2=0 to x=1/2`

Vậy `B_(min)=11/4⇔x=1/2`