$\dfrac{42}{x+2}+\dfrac{20}{x-2}=5$
$\leftrightarrow 42(x-2)+20(x+2)=5(x+2)(x-2)$
$\leftrightarrow 42x-84+20x+40=5(x^2-4)$
$\leftrightarrow 42x-84+20x+40=5x^2-20$
$\leftrightarrow 62x-44=5x^2-20$
$\leftrightarrow 62x-44-5x^2+20=0$
$\leftrightarrow -5x^2+62x-24=0$
$\leftrightarrow 5x^2-62x+24=0$
$\leftrightarrow 5x^2-60x-2x+24=0$
$\leftrightarrow 5x(x-12)-2(x-12)=0$
$\leftrightarrow (x-12)(5x-2)=0$
$\leftrightarrow \left[ \begin{array}{l}x-12=0\\5x-2=0\end{array} \right.$
$\leftrightarrow \left[ \begin{array}{l}x=12\\x=\dfrac{2}{5}\end{array} \right.$
Vậy `S={12;2/5}`
