\(A=\dfrac{2010x+2680}{x^2+1}=\dfrac{-335x^2-335+335x^2+2010x+3015}{x^2+1}\)
\(A=\dfrac{-335\left(x^2+1\right)}{x^2+1}+\dfrac{335\left(x^2+6x+9\right)}{x^2+1}\)
\(A=-335+\dfrac{335\left(x+3\right)^2}{x^2+1}\)
Do \(\dfrac{335\left(x+3\right)^2}{x^2+1}\ge0\forall x\Rightarrow A\ge-335\)
\(\Rightarrow A_{min}=-335\) khi \(x+3=0\Leftrightarrow x=-3\)