tìm x: 80/(x+4)+80/(x-4)=25/3

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

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

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

$⇔ 240.(x -4 + x +4) = 25.(x² -16)$

$⇔ 240.2x = 25x² -400$

$⇔ -25x² +480x +400 = 0$

$⇔ -(5x -48)² +2704 = 0$

$⇔ (52 -5x +48).(52 +5x -48) = 0$

$⇔ (100 -5x). (4 +5x) = 0$

$⇔ \left[ \begin{array}{l}100 -5x=0\\4 +5x=0\end{array} \right. ⇔ \left[ \begin{array}{l}x=20\\x=\frac{-4}{5}\end{array} \right.$

$Vậy$ $S =$ {$20; \frac{-4}{5}$}