\(\dfrac{x^3}{x^2-4}-\dfrac{x}{x-2}-\dfrac{2}{x+2}\)
\(=\dfrac{x^3}{\left(x-2\right)\left(x+2\right)}-\dfrac{x\left(x+2\right)}{\left(x+2\right)\left(x-2\right)}-\dfrac{2\left(x-2\right)}{\left(x+2\right)\left(x-2\right)}\)
\(=\dfrac{x^3-x^2-2x-2x+4}{\left(x+2\right)\left(x-2\right)}\)
\(=\dfrac{x^3-x^2-4x+4}{\left(x-2\right)\left(x+2\right)}\)
\(=\dfrac{\left(x-1\right)\left(x^2-4\right)}{\left(x-2\right)\left(x+2\right)}\)
\(=x-1\)
b) \(C=0\Rightarrow x-1=0\Rightarrow x=1\)
c) C luôn dương => \(C\ge0\Rightarrow x-1\ge0\Rightarrow x\ge1\)