a/
\(\Leftrightarrow x^2-2x+4-4=0\\ \Leftrightarrow x\left(x-2\right)=0\)
\(\Leftrightarrow x=0;x-2=0\)
\(\Leftrightarrow x=0;x=2\)
b/
\(\Leftrightarrow\left(x-3\right)\left(x+3\right)-2x\left(x-3\right)=0\)
\(\Leftrightarrow\left(x-3\right)\left(x+3-2x\right)=0\)
\(\Leftrightarrow\left(x-3\right)\left(3-x\right)=0\)
\(\Rightarrow x=3\)
\(a,x^2-2.\left(x-2\right)=4\\ \Leftrightarrow x^2-2x+4-4=0\\ \Leftrightarrow x^2-2x=0\\ \Leftrightarrow x.\left(x-2\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\x-2=0\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\x=2\end{matrix}\right.\\ \Rightarrow S=\left\{0;2\right\}\\ b,x^2-9-2x\left(x-3\right)=0\\ \Leftrightarrow x^2-4x^2+6x-9=0\\ \Leftrightarrow-3x^2+6x-9=0\\ \Leftrightarrow x^2-2x+3=0\\ \Leftrightarrow\left(x^2-2x+1\right)+2=0\\ \Leftrightarrow\left(x-1\right)^2=-2\left(vô.lí\right)\\ \Rightarrow Pt.vô.nghiệm\)
