Tìm GTNN biết : a) A= |x-5/7|-19 b) B= (x-3/4)^2+ 2020 Giúp mik lẹ nha

a,

$|x-\dfrac{5}{7}|\ge 0$

$\Rightarrow A\ge -19$

$\min A=-19\Leftrightarrow x=\dfrac{5}{7}$

b,

$(x-\dfrac{3}{4})^2\ge 0$

$\Rightarrow B\ge 2020$

$\min B=2020\Leftrightarrow x=\dfrac{3}{4}$