Bonsoir,
[tex] \dfrac{400}{250-x} = \dfrac{400}{250+x}+ \dfrac{2}{3} \\\\
\dfrac{400}{250-x} - \dfrac{400}{250+x}= \dfrac{2}{3} \\\\
\dfrac{400*(250+x)}{250^2-x^2}- \dfrac{400*(250-x)}{250^2-x^2}= \dfrac{2}{3} \\\\
\dfrac{400*(250+x-(250-x))}{62500-x^2} =\dfrac{2}{3} \\\\
\dfrac{400*(2*x)}{62500-x^2} =\dfrac{2}{3} \\\\
3*(800*x) =2*{62500-x^2} \\\\
x^2+1200x-62500=0\\\\
\Delta=1200^2+4*62500=1690000=1300^2\\\\
x= \dfrac{-1200-1300}{2} \ ou\ x= \dfrac{-1200+1300}{2}\\\\
x=-1250\ ou\ x=50\\\\
[/tex]