a,\(Đkxđ:x\ge3\)
Ta có:
\(\sqrt{\left(x-3\right)^2}=3-x\)
\(\Leftrightarrow|x-3|=3-x\)
\(\Leftrightarrow x-3=\left[{}\begin{matrix}x-3\\3-x\end{matrix}\right.\)
\(TH1:x-3=x-3\Leftrightarrow0x=0\)
\(\Rightarrow\)\(x\in R\) và \(x\ge3\)
\(TH2:x-3=3-x\Leftrightarrow2x=6\Leftrightarrow x=3\)( ko thỏa mãn điều kiện)
vậy \(\left\{x\in R/x\ge3\right\}\)
b, \(Đkxđ:x\le\dfrac{5}{2}\)
Ta có:
\(\sqrt{25-20x+4x^2}+2x=5\)
\(\Leftrightarrow\sqrt{\left(5-2x\right)^2}+2x=5\)
\(\Leftrightarrow\left|5-2x\right|=5-2x\)
\(\Leftrightarrow\left[{}\begin{matrix}5-2x=5-2x\\5-2x=2x-5\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}0x=0\\4x=10\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x\in R\\x=\dfrac{5}{2}\left(tmđk\right)\end{matrix}\right.\)
Vậy \(\left\{x\in R/x\le\dfrac{5}{2}\right\}\)