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* Hướng dẫn giải

Ta có

\(\begin{array}{c}\left\{ {\begin{array}{*{20}{c}}{{u_2} - {u_3} + {u_5} = 10}\\{{u_4} + {u_6} = 26}\end{array}} \right. \\ \Leftrightarrow \left\{ \begin{array}{l}{u_1} + d - {u_1} - 2d + {u_1} + 4d = 10\\{u_1} + 3d + {u_1} + 5d = 26\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}{u_1} + 3d = 10\\2{u_1} + 8d = 26\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}{u_1} = 1\\d = 3\end{array} \right.\end{array}\)