\(\left\{{}\begin{matrix}u_6+u_9=-7\\2u_4-u_5=1\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}u_1.q^5+u_1.q^8=-7\\2u_1q^3-u_1q^4=1\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}u_1q^3\left(q^2+q^5\right)=-7\\u_1q^3\left(2-q\right)=1\end{matrix}\right.\)
\(\Rightarrow q^5+q^2=-7\left(2-q\right)\)
\(\Rightarrow\left(q+2\right)\left(q^4-2q^3+4q^2-7q+7\right)=0\)
\(\Rightarrow q=-2\)
\(\Rightarrow u_1=-\dfrac{1}{32}\)
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