$\Leftrightarrow x^2 +\dfrac{25x^2}{(x+5)^2} = 11$
$\Leftrightarrow \dfrac{x^2}{x+5} \bigg ((x+5) +\dfrac{25}{x+5} \bigg ) = 11$
$\Leftrightarrow \dfrac{x^2}{x+5} \bigg (\dfrac{(x+5)^2 +25}{x+5} \bigg ) = 11$
$\Leftrightarrow \dfrac{x^2}{x+5} \bigg (\dfrac{x^2}{x+5} + 5 \bigg ) = 11$
Đặt $\dfrac{x^2}{x+5} =t$ coi như xong nhé