\(\frac{80}{x+4}+\frac{80}{x-4}=\frac{25}{3}\left(ĐkXĐ:x\ne\pm4\right)\\ \Leftrightarrow\frac{3.80\left(x-4\right)+3.80\left(x+4\right)-25.\left(x^2-16\right)}{\left(x^2-16\right).3}=0\\ \Leftrightarrow\frac{240x-960+240x+960-25x^2+400}{3.\left(x^2-16\right)}=0\\ \Leftrightarrow\frac{-25x^2+480x+400}{3.\left(x^2-16\right)}=0\\ \Leftrightarrow\frac{-25.\left(x+\frac{4}{5}\right)\left(x-20\right)}{3.\left(x^2-16\right)}=0\\ \Leftrightarrow\left[{}\begin{matrix}x+\frac{4}{5}=0\\x-20=0\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=-\frac{4}{5}\left(Nhận\right)\\x=20\left(Nhận\right)\end{matrix}\right.\\ \Rightarrow S=\left\{-\frac{4}{5};20\right\}\)
