$\displaystyle \begin{array}{{>{\displaystyle}l}} B=\frac{3\sqrt{x} +1}{x+2\sqrt{x} -3} -\frac{2}{\sqrt{x} +3}\\ Dk: x >0 \\ x\#1 \\ B=\frac{3\sqrt{x} +1}{x+3\sqrt{x} -\sqrt{x} -3} -\frac{2}{\sqrt{x} +3}\\ B=\frac{3\sqrt{x} +1-2\left(\sqrt{x} -1\right)}{\left(\sqrt{x} +3\right)\left(\sqrt{x} -1\right)}\\ B=\frac{\sqrt{x} +3}{\left(\sqrt{x} +3\right)\left(\sqrt{x} -1\right)} =\frac{1}{\sqrt{x} -1} \\ Với x-4\sqrt{x} +4 \leqslant 0 \\ \Longrightarrow \left(\sqrt{x} -2 \right)^{2} \leqslant 0 . Vô lí vì \left(\sqrt{x} -2\right)^{2} \geqslant 0 \end{array}$
thõa mãn dấu = xảy ra khi x=4
Thay x=4 vào B=$\frac{1}{\sqrt{4}-1}=1$
