`x+y=a`
`⇒x^2+y^2+2xy=a^2`
`⇒2xy=0`
\(⇒\left[ \begin{array}{l}x=0\\y=0\end{array} \right.\)
\(⇒\left[ \begin{array}{l}y=a\\x=a\end{array} \right.\)
\(⇒\left[ \begin{array}{l}Q=0+a^{2020}=a^{2020}\\Q=a^{2020}+0=a^{2020}\end{array} \right.\)
Vậy `Q=a^{2020}`