$\left | x + 3 \right | + \left | x - 5 \right | = \left | x + 3 \right | + \left | 5 - x \right | \geq \left | x + 3 + 5 - x \right | = \left | 8 \right | = 8$
Dấu "=" xảy ra khi $\left ( x + 3 \right )\left ( 5 - x \right ) \geq 0 \Leftrightarrow -3 \leq x \leq 5$
Vậy $minB = 8$ khi $-3 \leq x \leq 5$