$\frac{80}{x+4}$ + $\frac{80}{x-4}$ = $\frac{25}{3}$

$\frac{80}{x +4} + \frac{80}{x -4} = \frac{25}{3}$ $(ĐKXĐ:$ $x \neq ±4)$

$⇔ \frac{240.(x -4) +240.(x +4)}{3.(x -4).(x +4)} = \frac{25.(x +4).(x -4)}{3.(x +4).(x -4)}$ 

$⇒ 240.(x -4) +240.(x +4) = 25.(x +4).(x -4)$

$⇔ 240x -960 +240x +960 = 25x² -400$

$⇔ -25x² +480x +400 = 0$

$⇔ -25x.(x -20) -20.(x -20) = 0$
$⇔ (x -20).(-25x -20) = 0$

$⇔ \left[ \begin{array}{l}x -20=0\\-25x -20=0\end{array} \right. ⇔ \left[ \begin{array}{l}x=20\\x=\frac{-4}{5}\end{array} \right.$ $(T/m$ $đkxđ)$

$Vậy$ $S =$ {$20; \frac{-4}{5}$}