`A = x^2 - 2x + 6`
` = (x^2-2x+1) + 5`
`= (x-1)^2 + 5 \ge 5 \forall x`
Dấu `=` xảy ra `<=>x-1=0<=>x=1`
Vậy `\text{Min}_A = 5 <=>x=1`
``
`B = m^2 - m + 1`
` = (m^2 - m + 1/4) + 3/4`
`= (m-1/2)^2 + 3/4 \ge 3/4 \forall x`
Dấu `=` xảy ra `<=>m-1/2=0<=>m=1/2`
Vậy `\text{Min}_B = 3/4 <=>m=1/2`
``
`C = x^2 + 3x`
` = (x^2 + 3x + 9/4) - 9/4`
` = (x+3/2)^2 - 9/4 \ge -9/4 \forall x`
Dấu `=` xảy ra `<=>x+3/2=0<=>x=-3/2`
Vậy `\text{Min}_C = -9/4 <=>x=-3/2`
