Cho dãy số \(\left(u_{n}\right) \text { có } \mathrm{d}=-2 ; \mathrm{S}_{8}=72\), Tính \(u_1\)

* Hướng dẫn giải

Ta có \(\left\{\begin{array}{l} S_{n}=\frac{n\left(u_{1}+u_{n}\right)}{2} \\ d=\frac{u_{n}-u_{1}}{n-1} \end{array} \Rightarrow\left\{\begin{array}{l} u_{1}+u_{8}=2 S_{8}: 8 \\ u_{8}-u_{1}=7 d \end{array} \Rightarrow\left\{\begin{array}{l} u_{8}+u_{1}=18 \\ u_{8}-u_{1}=-14 \end{array} \Rightarrow u_{1}=16\right.\right.\right.\)