`A=y^2-4y+9`
`A=(y^2-4y+4)+5`
`A=(y-2)^2+5>=5`
Dấu = xảy ra khi `y=2`
Vậy `A_(min)=5<=>y=2`
`B=x^2-x+1`
`B=x^2-2.x. 1/2+1/4+3/4`
`B=(x-1/2)^2+3/4>=3/4`
Dấu = xảy ra khi `x=1/2`
Vậy `B_(min)=3/4<=>x=1/2`
`C=2x^2-6x`
`C=2(x^2-3x)`
`C=2(x^2-2.x. 3/2+9/4-9/4)`
`C=2(x-3/2)^2-9/2>=-9/2`
Dấu = xảy ra khi `x=3/2`
Vậy `C_(min)=-9/2<=>x=3/2`
