Đáp án:
`a) 4x(x-20)-x+20=0`
`<=>4x(x-20)-(x-20)=0`
`<=>(x-20)(4x-1)=0`
`<=>`\(\left[ \begin{array}{l}x-20=0\\4x-1=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=20\\x=1/4\end{array} \right.\)
`b) (x+8)^2-121=0`
`<=>(x+8-11)(x+8+11)=0`
`<=>(x-3)(x+19)=0`
`<=>`\(\left[ \begin{array}{l}x-3=0\\x+19=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=3\\x=-19\end{array} \right.\)
`c) 4x^2-12x=-9`
`<=>4x^2-12x+9=0`
`<=> (2x)^2-2.2x.3+3^2=0`
`<=> (2x-3)^2=0`
`<=> 2x-3=0`
`<=> x=3/2`
