`B = 2x - x^2 - 2`
`= -x^2 + 2x - 1 - 1`
`= - (x^2 - 2x + 1) - 1`
`= - (x^2 - x - x + 1) - 1`
`= - [x(x - 1) - (x - 1)] - 1`
`= - [(x - 1)(x - 1)] - 1`
`= - (x - 1)^2 - 1`
Với mọi `x`, ta có:
`(x - 1)^2 >= 0`
`<=> - (x - 1)^2 <= 0`
`<=> - (x - 1)^2 - 1 <= -1`
`<=> B <= -1`
Dấu "`=`" xảy ra
`<=> x - 1 = 0`
`<=> x = 1`
Vậy `Max B = -1 <=> x = 1`