Đáp án:
`A=x^2+2x+5`
`=x^2+2x+1+4`
`=(x+1)^2+4`
Vì `(x+1)^2>=0AAx`
`=>(x+1)^2+4>=4AAx`
`=>A>=4`
Dấu "=" xảy ra khi `x+1=0<=>x=-1`
Vậy `min_A=4<=>x=-1`.
`B=4x^2-4x+3`
`=4x^2-4x+1+2`
`=(2x-1)^2+2`
Vì `(2x-1)^2>=0AAx`
`=>(2x-1)^2+2>=2`
`=>B>=2`
Dấu "=" xảy ra khi `2x-1=0<=>x=1/2`
Vậy `min_B=2<=>x=1/2`.