Cho cấp số nhân $\left( {{u_n}} \right)$ biết \(...

D

- Hướng dẫn giải

$\left\{ \begin{array}{l}{u_1} + {u_2} + {u_3} = 31\\{u_1} + {u_3} = 26\end{array} \right. \Rightarrow \left\{ \begin{array}{l}{u_2} = 5\\{u_1} + {u_3} = 26\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}q{u_1} = 5\\{u_1} + {q^2}{u_1} = 26\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}q{u_1} = 5\\{u_1} + 5q = 26\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}{u_1} = \frac{5}{q}\\\frac{5}{q} + 5q = 26\end{array} \right.$

$\Leftrightarrow \left[ \begin{array}{l}q = \frac{1}{5};\,\,{u_1} = 25\\q = 5;\,\,{u_1} = 1\end{array} \right.$