a. \(A=\left(\dfrac{1}{x-1}-\dfrac{1}{x+1}\right):\dfrac{x}{x^2-1}\)
\(A=\left(\dfrac{x+1-x+1}{\left(x+1\right)\left(x-1\right)}\right).\dfrac{\left(x-1\right)\left(x+1\right)}{x}\)
\(A=\dfrac{2}{\left(x+1\right)\left(x-1\right)}.\dfrac{\left(x-1\right)\left(x+1\right)}{x}\)
\(A=\dfrac{2}{x}\)
b.
Ta có:
\(A=\dfrac{2}{x}=\dfrac{1}{2}\Rightarrow x=4\)
c.
\(A=\dfrac{2}{x}< 0\Rightarrow x< 0\)