Cho cấp số cộng

- Hướng dẫn giải

Ta có:

$\begin{array}{l}\left\{\begin{array}{l}{u}_1+u_7=26\\ {u_2}^2+{u_6}^2=466\end{array}\right.\iff \left\{\begin{array}{l}{u}_1+u_1+6d=26\\ {\left(u_1+d\right)}^2+{\left(u_1+5d\right)}^2=466\end{array}\right.\\ \iff \left\{\begin{array}{l}{u}_1=13-3d\left(1\right)\\ {\left(u_1+d\right)}^2+{\left(u_1+5d\right)}^2=466\left(2\right)\end{array}\right..\end{array}$

Thay (1) và (2) ta được:

$$\begin{gathered} \begin{array}{l}{\left(13-2d\right)}^2+{\left(13+2d\right)}^2=466\iff 8d^2+338=466\\ \iff 8d^2=128\iff d^2=16\end{array} \\ \iff \left[\begin{array}{l}d=4\Rightarrow u_1=1\\ d=-4\Rightarrow u_1=25\end{array}\right. \end{gathered}$$ 

Chọn đáp án A