\(A=\frac{x}{\left(x+2020\right)^2}=\frac{8080x}{8080\left(x+2020\right)^2}=\frac{x^2+4040x+2020^2-x^2+4040x-2020^2}{8080\left(x+2020\right)^2}\)
\(=\frac{\left(x+2020\right)^2-\left(x-2020\right)^2}{8080\left(x+2020\right)^2}=\frac{1}{8080}-\frac{\left(x-2020\right)^2}{8080\left(x+2020\right)^2}\le\frac{1}{8080}\)
\(A_{max}=\frac{1}{8080}\) khi \(x=2020\)