`***`Lời giải`***`
`D=(\frac{4 \sqrt{x}}{3- \sqrt{x}}-\frac{8x}{9-x}):(\frac{ \sqrt{x}-4}{x+3 \sqrt{x}}+\frac{1}{ \sqrt{x}})`
ĐKXĐ: `xne9;x≥0`
`=\frac{4 \sqrt{x}(3+ \sqrt{x})- 8x}{9-x}:(\frac{ \sqrt{x}-4}{\sqrt{x}(\sqrt{x}+3)}+\frac{1}{ \sqrt{x}})`
`=\frac{4 \sqrt{x}(3+ \sqrt{x})- 8x}{9-x}:\frac{ \sqrt{x}-4+\sqrt{x}+3}{\sqrt{x}(\sqrt{x}+3)}`
`=\frac{12 \sqrt{x}+4x- 8x}{9-x}:\frac{ \sqrt{x}-4+\sqrt{x}+3}{\sqrt{x}(\sqrt{x}+3)}`
`=\frac{12 \sqrt{x}-4x}{9-x}:\frac{2\sqrt{x}-1}{\sqrt{x}(\sqrt{x}+3)}`
`=\frac{4\sqrt{x}(3-\sqrt{x})}{9-x}:\frac{2\sqrt{x}-1}{\sqrt{x}(\sqrt{x}+3)}`
`=\frac{4\sqrt{x}(3-\sqrt{x})}{9-x}.\frac{\sqrt{x}(\sqrt{x}+3) }{2\sqrt{x}-1}`
`=\frac{4x }{2\sqrt{x}-1}`
Vậy `B=\frac{4x }{2\sqrt{x}-1}` với `xne9;x≥0`
