Đáp án:
`a) 5x(x-20)-x+20=0`
`<=> 5x(x-20)-(x-20)=0`
`<=> (x-20)(5x-1)=0`
`<=>` \(\left[ \begin{array}{l}x-20=0\\5x-1=0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=20\\x=\frac{1}{5}\end{array} \right.\)
`b) x^2(x-3)+12-4x=0`
`<=> x^2(x-3)-4(x-3)=0`
`<=> (x-3)(x^2-4)=0`
`<=> (x-3)(x-2)(x+2)=0`
`<=>`\(\left[ \begin{array}{l}x-3=0\\x-2=0\\x+2=0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=3\\x=2\\x=-2\end{array} \right.\)
`c) x^2-x-6=0`
`<=> x^2-3x+2x-6=0`
`<=> x(x-3)+2(x-3)=0`
`<=> (x-3)(x+2)=0`
`<=>` \(\left[ \begin{array}{l}x-3=0\\x+2=0\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=3\\x=-2\end{array} \right.\)
