$B = \bigg(\dfrac{15 - \sqrt x}{x - 25} + \dfrac{2}{\sqrt x + 5}\bigg): \dfrac{\sqrt x + 1}{\sqrt x - 5} (ĐKXĐ: x \geq 0;x \neq 25)$
$=\bigg(\dfrac{15 -\sqrt x}{(\sqrt x -5)(\sqrt x +5)} + \dfrac{2}{\sqrt x +5}\bigg) . \dfrac{\sqrt x- 5}{|sqrt x +1}$
$=\dfrac{15 - \sqrt x + 2(\sqrt x- 5)}{(\sqrt x -5)(\sqrt x +5)} . \dfrac{\sqrt x - 5}{\sqrt x +1}$
$=\dfrac{15 - \sqrt x + 2\sqrt x - 10}{(\sqrt x -5)(\sqrt x +5)} . \dfrac{\sqrt x - 5}{\sqrt x +1}$
$=\dfrac{\sqrt x + 5}{(\sqrt x -5)(\sqrt x +5)} . \dfrac{\sqrt x - 5}{\sqrt x +1}$
$=\dfrac{1}{\sqrt x + 1}$
