a/ $\Big|x-\dfrac{1}{2}\Big|\ge 0$
$\to$ Dấu "=" xảy ra khi $x-\dfrac{1}{2}=0$
$\leftrightarrow x=\dfrac{1}{2}$
$\to \min A=0$ khi $x=\dfrac{1}{2}$
b/ $\Big|x+\dfrac{3}{4}\Big|\ge 0$
$\to \Big|x+\dfrac{3}{4}\Big|+2\ge 2$
$\to$ Dấu "=" xảy ra khi $x+\dfrac{3}{4}=0$
$\leftrightarrow x=-\dfrac{3}{4}$
$\to \min B=2$ khi $x=-\dfrac{3}{4}$