`A=x^2-4x+9`
`=x^2-4x+4+5`
`=(x-2)^2+5>=5`
Dấu "=" xảy ra khi `x-2=0<=>x=2`
Vậy `GTNNNN=5` khi `x-2=0<=>x=2`
`B=-2x^2+6x+7`
`=-2x^2+2.2.x. 3/2 -2. 9/4+23/2`
`=-2(x^2-2.x. 3/2+9/4)+23/2`
`=-2(x-3/2)^2+23/2<=23/2`
Dấu "=" xảy ra khi `x-3/2=0<=>x=3/2`
Vậy `GTLNN=23/2` khi `x-3/2=0<=>x=3/2`