Đáp án: `S={2;13/4}`
Giải thích các bước giải:
`(x+7)(8x-12) = (4x +1)(5x-6)`
`<=> 8x² -12x +56x -84 = 20x² -24x +5x -6`
`<=> 8x² +44x -84 = 20x² -19x -6`
`<=> 12x² -63x +78=0`
`<=> x² -(21)/4 x +(13)/2=0`
`<=> x² - 2.(21)/8.x + ((21)/(8))^2 -(25)/(64)=0`
`<=> (x -(21)/(8))^2 = (25)/(64)`
`<=>` \(\left[ \begin{array}{l}x- \dfrac{21}{8} = \dfrac58\\x-\dfrac{21}{8} = -\dfrac58\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=\dfrac{21}{8} + \dfrac58\\x=\dfrac{21}{8} -\dfrac58\end{array} \right.\)
`<=>` \(\left[ \begin{array}{l}x=\dfrac{13}{4}\\x= 2\end{array} \right.\)
Vậy `S={2;13/4}`
