9. 记 $S_{n}$ 为等差数列 $\left\{a_{n}\right\}$ 的前 $n$ 项和. 已知 $S_{4}=0, a_{5}=5$, 则
Gaokao · Math · previous-year question
- A.$a_{n}=2 n-5$correct
- B.$a_{n}=3 n-10$
- C.$S_{n}=2 n^{2}-8 n$
- D.$S_{n}=\frac{1}{2} n^{2}-2 n$
Answer
A. $a_{n}=2 n-5$
Explanation
【详解】由题知, $\left\{\begin{array}{l}S_{4}=4 a_{1}+\frac{d}{2} \times 4 \times 3=0 \\ a_{5}=a_{1}+4 d=5\end{array}\right.$, 解得 $\left\{\begin{array}{l}a_{1}=-3 \\ d=2\end{array}, \therefore a_{n}=2 n-5\right.$, 故选 A.
Practice more Gaokao questions
Answer thousands more real Gaokao previous-year questions free, get graded instantly, and climb the global ranked leaderboard.
Practice Gaokao free →More Gaokao Math questions
- 设集合 A=\{x \mid x \geq 1\}, B=\{x \mid-1<x<2\}, 则 A \cap B= (\quad)…
- 已知 a \in R,(1+a i) i=3+i, ( i 为虚数单位), 则 a=(\quad)…
- 已知非零向量 \vec{a}, \vec{b}, \vec{c}, 则“ \vec{a} \cdot \vec{c}=\vec{b} \cdot \vec{c} ”是“ \vec{…
- 若实数 x, y 满足约束条件 \left\{\begin{array}{l}x+1 \geq 0 \\ x-y \leq 0 \\ 2 x+3 y-1 \leq0\end{arr…
- 已知 a, b \in \mathrm{R}, a b>0, 函数 f(x)=a x^{2}+b(x \in \mathrm{R}). 若 f(s-t), f(s), f(s+t)…
- 已知数列 \left\{a_{n}\right\} 满足 a_{1}=1, a_{n+1}=\frac{a_{n}}{1+\sqrt{a_{n}}}\left(n \in \mat…
- 设 z=\frac{1-i}{1+i}+2 i, 则 |z|=(\qquad)…
- 已知集合 A=\left\{x \mid x^{2}-x-2>0\right\}, 则 C_{R} A=（ \qquad ）…