Cho cấp số cộng (un) th...

A

- Hướng dẫn giải

Chọn A

Ta có $\left\{\begin{array}{c}{u}_2-u_3+u_5=10\\ u_4+u_6=26\end{array}\right.$

$\begin{array}{l}\iff \left\{\begin{array}{l}{u}_1+d-(u_1+2d)+u_1+4d=10\\ u_1+3d+u_1+5d=26\end{array}\right.\\ \iff \left\{\begin{array}{l}{u}_1+3d=10\\ 2u_1+8d=26\end{array}\right.\end{array}$

$\iff u_1=\mathrm{1,}d=3$