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Ta có: $\left\{ \begin{array}{l} u_1-u_3+u_5=-85\\u_1+u_7=-425 \end{array} \right.$
$\Leftrightarrow \left\{ \begin{array}{l} u_1-u_1q^2+u_1q^4=-85\\u_1+u_1q^6=-425 \end{array} \right.$
$\Leftrightarrow \left\{ \begin{array}{l} u_1(1-q^2+q^4)=-85\\u_1(1+q^6)=-425 \end{array} \right.$
$\Leftrightarrow \left\{ \begin{array}{l} u_1(1-q^2+q^4)=-85\\\frac{u_1(1+q^6)}{u_1(1-q^2+q^4)}=5 \end{array} \right.$
$\Leftrightarrow \left\{ \begin{array}{l} u_1(1-q^2+q^4)=-85\\ 1+q^2=5 \end{array} \right.$
$\Leftrightarrow \left\{ \begin{array}{l} q=\pm2\\ u_1=\frac{-85}{13} \end{array} \right.$
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