\(\left\{{}\begin{matrix}u_2+u_4+u_6=-42\\u_3+u_5=20\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}u_1q+u_1q^3+u_1q^5=-42\\u_1q^2+u_1q^4=20\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}u_1q\left(1+q^2+q^4\right)=-42\\u_1q\left(q+q^3\right)=20\end{matrix}\right.\)
\(\Rightarrow\dfrac{1+q^2+q^4}{q+q^3}=\dfrac{-42}{20}\)
\(\Rightarrow10q^4+21q^3+10q^2+21q+10=0\)
\(\Rightarrow\left(2q^2+5q+2\right)\left(5q^2-2q+5\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}q=-2\Rightarrow u_1=1\\q=-\dfrac{1}{2}\Rightarrow u_1=64\end{matrix}\right.\)
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