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Câu hỏi: Cho cấp số công $\left(u_n\right):\left\{\begin{array}{l}6 u_1+5 u_5=28 \\ S_4=14\end{array}\right.$. Chọn đáp án đúng
A. $\left\{\begin{array}{l}u_1=6 \\ d=2\end{array}\right.$.
B. $\left\{\begin{array}{l}u_1=8 \\ d=-3\end{array}\right.$.
C. $\left\{\begin{array}{l}u_1=2 \\ d=6\end{array}\right.$.
D. $\left\{\begin{array}{l}u_1=-3 \\ d=8\end{array}\right.$.
A. $\left\{\begin{array}{l}u_1=6 \\ d=2\end{array}\right.$.
B. $\left\{\begin{array}{l}u_1=8 \\ d=-3\end{array}\right.$.
C. $\left\{\begin{array}{l}u_1=2 \\ d=6\end{array}\right.$.
D. $\left\{\begin{array}{l}u_1=-3 \\ d=8\end{array}\right.$.
Ta có $\left\{\begin{array}{l}6 u_1+5 u_5=28 \\ S_4=14\end{array} \Leftrightarrow\left\{\begin{array}{l}6 u_1+5\left(u_1+4 d\right)=28 \\ 2\left(2 u_1+3 d\right)=14\end{array} \Leftrightarrow\left\{\begin{array}{l}11 u_1+20 d=28 \\ 2 u_1+3 d=7\end{array} \Leftrightarrow\left\{\begin{array}{l}u_1=8 \\ d=-3\end{array}\right.\right.\right.\right.$.
Đáp án B.