`text{Ta có}`
`A`
`= x^2 - 2x + 5`
`= x^2 - 2x + 1 + 4`
`= (x - 1)^2 + 4`
`text{Ta có}`
`(x - 1)^2 >= 0` `text{với}` `AA x in RR`
`-> (x - 1)^2 + 4 >= 4` `text{với}` `AA x in RR`
`-> A >= 4` `text{với}` `AA x in RR`
`text{Dấu}`"`=`" `text{xảy ra}`
`-> x - 1 = 0`
`-> x = 1`
`text{Vậy}` `A_{min} = 4` `text{khi}` `x = 1`