\(x^4-2x^2=400x+9999=0\)
\(\Leftrightarrow x^4+11x^3+119x^2+909x-11\left(x^3+11x^3+119x+909\right)=0\)
\(\Leftrightarrow\left(x-11\right)\left(x^3+11x^2+119x+909\right)=0\)
\(\Leftrightarrow\left(x-11\right)\left(x^3+2x^2+101x+9x^2+18x+909\right)=0\)
\(\Leftrightarrow\left(x-11\right)\left(x+9\right)\left(x^2+2x+101\right)=0\)
\(\Rightarrow\orbr{\begin{cases}11\\-9\end{cases}}\)