Giải thích các bước giải:
4) $\dfrac{2-\dfrac{a}{x}}{2+\dfrac{a}{x}}$
$=\dfrac{2x-a}{2x+a}$
5) $\dfrac{\dfrac{x}{x-1}-\dfrac{x+1}{x}}{\dfrac{x}{x+1}-\dfrac{x-1}{x}}$
$=\dfrac{\dfrac{x^2-(x-1)(x+1)}{x(x-1)}}{\dfrac{x^2-(x-1)(x+1)}{x(x+1)}}$
$=\dfrac{x+1}{x-1}$