\(\Leftrightarrow\left(x+y\right)^2=100\)
\(\Leftrightarrow\left[{}\begin{matrix}x+y=10\\x+y=-10\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=10-y\\x=-10-y\end{matrix}\right.\)
\(D=xy=\left[{}\begin{matrix}y\left(10-y\right)=10y-y^2\\y\left(-10-y\right)=-10y-y^2\end{matrix}\right.\)