Ta có :
\(\left\{{}\begin{matrix}u_{15}+u_7=60\\u_{12}+u_4=1170\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}u_1+14d+u_1+6d=60\\u_1+11d+u_1+3d=1170\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}2u_1+20d=60\\2u_1+14d=1170\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}-6d=1110\\2u_1+20d=60\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}d=-185\\u_1=30-10d\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}d=-185\\u_1=1880\end{matrix}\right.\)
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