Cho dãy số (un) có d = –2; S8 = 72. Tính u1 ?

* Hướng dẫn giải

Chọn B

Ta có $S_8=\frac{n}{2}.\text{[\hspace{0.17em}\hspace{0.17em}}2u_1+\text{\hspace{0.17em}\hspace{0.17em}}(n-1)d\text{]}$

$\begin{array}{l}\Rightarrow \text{ 72 = }\frac{8}{2}.\text{\hspace{0.33em}\hspace{0.17em}[}2.u_1+\text{}(8-1).(-2)\text{]}\\ \iff {\text{72 = 4 . (2u}}_1\text{}-14)\\ \iff 18=2u_1-14\\ \iff 2u_1=32\\ \iff u_1=16\end{array}$