Cho cấp số cộng

D

- Hướng dẫn giải

Đáp án D

Ta có  $\left\{\begin{array}{l}{u}_5+3u_3-u_2=-21\\ 3u_7-2u_4=-34\end{array}\right.\iff \left\{\begin{array}{l}{u}_1+4d+3\left(u_1+2d\right)-u_1-d=-21\\ 3\left(u_1+6d\right)-2\left(u_1+3d\right)=-34\end{array}\right.$

$\iff \left\{\begin{array}{l}3u_1+9d=-21\\ u_1+12d=-34\end{array}\right.\iff \left\{\begin{array}{l}{u}_1=2\\ d=-3\end{array}\right.\Rightarrow S_{15}=\frac{u_1+u_{15}}{2}.15=\frac{u_1+u_1+14d}{2}.15=-285$