c) Tìm x để A-1/B nhỏ hơn bằng -1/2

c) Ta có: $\frac{A-1}{B}=\left(\frac{\sqrt{x}-1}{\sqrt{x}+3}-1\right):\frac{\sqrt{x}+1}{\sqrt{x}+3}=\frac{\sqrt{x}-1-\sqrt{x}-3}{\sqrt{x}+3}.\frac{\sqrt{x}+3}{\sqrt{x}+1}=-\frac{4}{\sqrt{x}+1}$

Khi đó: $\frac{A-1}{B}\le -\frac{1}{2}\iff -\frac{4}{\sqrt{x}+1}+\frac{1}{2}\le 0\iff \frac{\sqrt{x}-7}{2\left(\sqrt{x}+1\right)}\le 0\iff \sqrt{x}-7\le 0\iff 0\le x\le 49$ 

(vì $2\left(\sqrt{x}+1\right)>0$).