$\text { $\frac{1}{x+1}$ -$\frac{1}{x+8}$ =$\frac{1}{14}$ }$
<=> $\frac{14(x+8)}{14(x+1)(x+8)}$ -$\frac{14(x+1)}{14(x+1)(x+8)}$ =$\frac{(x+1)(x+8)}{14(x+1)(x+8)}$
<=> $14(x+8) - 14(x+1) = (x+1)(x+8)$
<=> $ 14x + 112 -14x -14= x^2 +9x +8$
<=> $x^2 + 9x -90 =0$
<=> $x^2 + 15x -6x -90 =0$
<=> $x(x+15) -6(x+15)=0$
<=> $(x+15)(x-6) =0$
$\text{ <=> x+15 =0 hoặc x-6=0 }$
$\text{<=> x=-15 hoặc x=6}$
