ĐKXĐ: \(x\ge0\)
Ta có: \(\sqrt{x}+\sqrt{\frac{x}{9}}-\frac{1}{3}\sqrt{4x}=5\)
\(\Leftrightarrow\sqrt{x}+\sqrt{x}\cdot\frac{1}{3}-\frac{1}{3}\cdot2\sqrt{x}=5\)
\(\Leftrightarrow\sqrt{x}\cdot\frac{2}{3}=5\)
\(\Leftrightarrow\sqrt{x}=5:\frac{2}{3}=5\cdot\frac{3}{2}=\frac{15}{2}\)
\(\Leftrightarrow x=\frac{225}{4}\)
Vậy: \(S=\left\{\frac{225}{4}\right\}\)