Ta có
$\dfrac{1}{x-5x^2} - \dfrac{25x-15}{25x^2 - 1} = \dfrac{1}{x(1-5x)} - \dfrac{25x-15}{(5x-1)(5x+1)}$
$= \dfrac{1 + 5x}{x(1-5x)(1+5x)} + \dfrac{x(25x-15)}{x(1-5x)(1+5x)}$
$= \dfrac{1 + 5x + 25x^2 - 15x}{x(1-5x)(1+5x)}$
$= \dfrac{25x^2 - 10x + 1}{x(1-5x)(1+5x)}$
$= \dfrac{(1-5x)^2}{x(1-5x)(1+5x)}$
$= \dfrac{1-5x}{x(1+5x)}$