Bài `3:`
$A/B=B:A=\dfrac{2\sqrt{x}+1}{\sqrt{x}+1}:\dfrac{2\sqrt{x}+1}{3\sqrt{x}+1}$
$=\dfrac{2\sqrt{x}+1}{\sqrt{x}+1}.\dfrac{3\sqrt{x}+1}{2\sqrt{x}+1}$
$=\dfrac{3\sqrt{x}+1}{\sqrt{x}+1}$
$=3-\dfrac{2}{\sqrt{x}+1}$
Vì `\sqrt{x}≥0⇔\sqrt{x}+1≥1`
$⇔\dfrac{2}{\sqrt{x}+1}≤2$
Vì $\dfrac{2}{\sqrt{x}+1}\ne0$
$⇔3-\dfrac{2}{\sqrt{x}+1}<3$
Vậy `B/A<0`