Cho dãy số $\Large (u_n)$ có $\Large d=-2$; $\Large S_8=72$. Tính $\La

Cho dãy số $\Large (u_n)$ có $\Large d=-2$; $\Large S_8=72$. Tính $\Large u_1$?

• A.

  $\Large \Rightarrow u_1=16$.

• B.

  $\Large \Rightarrow u_1=-16$.

• C.

  $\Large \Rightarrow u_1=\dfrac{1}{16}$.

• D.

  $\Large \Rightarrow u_1=-\dfrac{1}{16}$.

Chọn A

Ta có: $\Large \left\{\begin{align} & S_n=\dfrac{n(u_1+u_n)}{2} \\ & d=\dfrac{u_n-u_1}{n-1} \end{align}\right.$ $\Large \Rightarrow \left\{\begin{align} & u_1+u_n=2S_8: 8 \\ & u_8-u_1=7d \end{align}\right.$ $\Large \Rightarrow \left\{\begin{align} & u_8+u_1=18 \\ & u_8-u_1=-14 \end{align}\right.$ $\Large \Rightarrow u_1=16$.