a)
ĐKXĐ: \(0\le x\le3\)
Ta có: \(\sqrt{x^2-x}=\sqrt{3-x}\)
\(\Leftrightarrow x^2-x=3-x\)
\(\Leftrightarrow x^2-x+x-3=0\)
\(\Leftrightarrow x^2-3=0\)
\(\Leftrightarrow x^2=3\)
hay \(x=\pm\sqrt{3}\)
Kết hợp ĐKXĐ, ta có: \(x=\sqrt{3}\)
Vậy: \(S=\left\{\sqrt{3}\right\}\)