高中 | 4.2.2 等差数列的前n项和公式 题目答案及解析

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高中 | 4.2.2 等差数列的前n项和公式题目答案及解析如下,仅供参考!

第四章 数列

4.2 等差数列

4.2.2 等差数列的前n项和公式

已知等差数列$\{a_{n}\}$的前$n$项和为$S_{n}.$若数列$\{b_{n}\}$满足:对任意的$n\in {\bf N}^{*}$,都有$a_{n}+b_{n}=n-1$,且${S}_{n}={b}_{n}^{2}$,则$a_{10}=(\qquad)$

["

$10$

","

$19$

","

$20$

","

$39$

"]
[["B"]]

$\because $ 等差数列$\{a_{n}\}$的前$n$项和为$S_{n}$,对任意的$n\in {\bf N}^{*}$,都有$a_{n}+b_{n}=n-1$,且${S}_{n}={b}_{n}^{2}$

$\therefore \begin{cases}{a}_{1}+{b}_{1}=0\\ {S}_{1}={a}_{1}={b}_{1}^{2}\end{cases}\Rightarrow \begin{cases}{a}_{1}=1\\ {b}_{1}=-1\end{cases}$$\begin{cases}{a}_{1}=0\\ {b}_{1}=0\end{cases}$

$\{a_{n}\}$公差为$d$,则$b_{n}=-a_{n}+n-1=(1-d)n+d-a_{1}-1$

${S}_{n}={b}_{n}^{2}$

${S}_{n}=\dfrac{d}{2}{n}^{2}+\left({a}_{1}-\dfrac{d}{2}\right)n$

$={(1-d)}^{2}{n}^{2}+2(1-d)(d-{a}_{1}-1)n+{(d-{a}_{1}-1)}^{2}$

对照系数得:$\begin{cases}\dfrac{d}{2}={(1-d)}^{2}\\ d-{a}_{1}-1=0\end{cases}\Rightarrow \begin{cases}\dfrac{d}{2}={(1-d)}^{2}\\ {a}_{1}=\dfrac{d}{2}\\ d-{a}_{1}-1=0\end{cases}$,解得$\begin{cases}d=2\\ {a}_{1}=1\end{cases}$

$\therefore a_{10}=a_{1}+9d=19$

故选:$\rm B$

高中 | 4.2.2 等差数列的前n项和公式题目答案及解析(完整版)

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