\(A=\dfrac{2021^2\left(2020-2019\right)}{\left(2020-1\right)\left(2020^3+1\right)}\cdot\dfrac{2019^2\left(2020^2+2021\right)}{2020^3-1}\)
\(A=\dfrac{2021^2}{2019\left(2020+1\right)\left(2020^2-2020\cdot1+1^2\right)}\cdot\dfrac{2019^2\left(2020^2+2021\right)}{\left(2020-1\right)\left(2020^2+2020\cdot1+1^2\right)}\)
\(A=\dfrac{2021^2\cdot2019^2\cdot\left(2020^2+2021\right)}{2019\cdot2021\left(2020^2-2019\right)\cdot2019\cdot\left(2020^2+2021\right)}\)
\(A=\dfrac{2021^2}{2021\left(2020^2-2019\right)}\)
\(A=\dfrac{2021}{2020^2-2019}\)