11. \(f\left(x\right)=x^2-2x=0\Rightarrow x\left(x-2\right)=0\Rightarrow\left[{}\begin{matrix}x=0\\x-2=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=0\\x=2\end{matrix}\right.\)
12. \(f\left(x\right)=\dfrac{2}{3}x-\dfrac{1}{3}x^2=0\Rightarrow\dfrac{1}{3}x\left(2-x\right)=0\Rightarrow\left[{}\begin{matrix}\dfrac{1}{3}x=0\\2-x=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=0\\x=2\end{matrix}\right.\)
13. \(f\left(x\right)=\left(2x-4\right)\left(x+1\right)=0\Rightarrow\left[{}\begin{matrix}2x-4=0\\x+1=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}2x=4\\x=-1\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=2\\x=-1\end{matrix}\right.\)
14. \(f\left(x\right)=x^2+1=0\Rightarrow x^2=-1\) (vô lý)
15. \(f\left(x\right)=\left(4x-1\right)\left(2x+3\right)=0\Rightarrow\left[{}\begin{matrix}4x-1=0\\2x+3=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}4x=1\\2x=-3\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}\text{x}=\dfrac{1}{4}\\x=-\dfrac{3}{2}\end{matrix}\right.\)
16. \(f\left(x\right)=\left(-5x+2\right)\left(x-7\right)=0\Rightarrow\left[{}\begin{matrix}-5x+2=0\\x-7=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}-5x=-2\\x=7\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=\dfrac{2}{5}\\x=7\end{matrix}\right.\)
17. \(f\left(x\right)=x^2-5x=0\Rightarrow x\left(x-5\right)=0\Rightarrow\left[{}\begin{matrix}x=0\\x-5=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=0\\x=5\end{matrix}\right.\)
18. \(f\left(x\right)=-4x^2+8x=0\Rightarrow-4x\left(x-2\right)=0\Rightarrow\left[{}\begin{matrix}-4x=0\\x-2=0\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x=0\\x=2\end{matrix}\right.\)
