












































































































































































































































































































































































































































































































































































































































































































































































































































































2025版《优化探究》高考数学总复习人教版数学配套课件
展开[学习要求] 1.熟练掌握等差数列、等比数列的前 n 项和公式,能够利 用公式求数列的前 n 项和. 2.掌握非等差、等比数列求和的几种常见方 法. 3.能在具体的问题情境中识别数列的等差关系或等比关系,并能用 相关知识解决与前 n 项和相关的问题.
知识点 数列前 n 项和的求法
(1)等差数列的前 n 项和公式
(2)等比数列的前 n 项和公式
①当 q =1时, Sn = na 1;
把数列的每一项分成两项或几项,使其转化为几个能求和的数列, 再求解.
把数列的通项拆成两项之差求和,正负相消剩下首尾若干项.
把数列分别正着写和倒着写再相加,即等差数列求和公式的推导过程的 推广.
主要用于一个等差数列与一个等比数列对应项相乘所得的数列的求和, 即等比数列求和公式的推导过程的推广.
一个数列的前 n 项和中,可两两结合求解,则称之为并项求和.形如 an = (-1) nf ( n )类型,可采用两项合并求解.
∵ an =(-1) n +1·(2 n +1),故 a 1=3, a 2=-5, a 3=7, a 4=-9,
故 S 2 023= a 1+ a 2+ a 3+…+ a 2 022+ a 2 023=3-5+7-9+…-4 045+4 047=-2×1 011+4 047=2 025.
4. 已知数列{ an }的前 n 项和为 Sn 且 an = n ·2 n ,则 Sn = .
因为 an = n ·2 n ,
所以 Sn =1×2+2×22+3×23+…+ n ×2 n ,①
所以2 Sn =1×22+2×23+3×24+…+ n ×2 n +1,②
( n -1)2 n +1+2
考点一 分组转化求和与并项求和
(1)记 bn = a 2 n ,写出 b 1, b 2,并求数列{ bn }的通项公式;
[解] (1)因为 bn = a 2 n ,所以 b 1= a 2= a 1+1=2, b 2= a 4= a 3+1= a 2+2+1= a 1+1+3=5.由题意得 a 2 n +1= a 2 n +2, a 2 n +2= a 2 n +1+1,所以 a 2 n +2= a 2 n +3,即 bn +1= bn +3,所以数列{ bn }是以2为首项,3为公差的等差数列,所以 bn =2+( n -1)×3=3 n -1, n ∈N*.
(2)求{ an }的前20项和.
1. 记数列{ an }的前 n 项和为 Sn ,已知 Sn =2 an -2 n +1.
(1)求数列{ an }的通项公式;
解:(1)当 n =1时,由 Sn =2 an -2 n +1,可得 a 1= S 1=2 a 1-2+1,即 有 a 1=1.当 n ≥2时, an = Sn - Sn -1=2 an -2 n +1-2 an -1+2( n -1)-1,即 an =2 an -1+2,可得 an +2=2( an -1+2),显然 an -1+2≠0,所以数列{ an +2}是首项为3,公比为2的等比数列,则 an +2=3·2 n -1, 即有 an =3·2 n -1-2.
考点二 裂项相消法求和
裂项相消法求数列{ an }的前 n 项和
一般是前边裂几项,后边就裂几项,直到发现被消去项的规律为止.
消项后前边剩几项,后边就剩几项,前边剩第几项,后边就剩倒数 第几项.
考点三 错位相减法求和
(2020·全国Ⅰ卷)设{ an }是公比不为1的等比数列, a 1为 a 2, a 3的 等差中项.
(1)求{ an }的公比;
[解] (1)设{ an }的公比为 q ,由题意得2 a 1= a 2+ a 3,即2 a 1= a 1 q + a 1 q 2,所以 q 2+ q -2=0,
解得 q =1(舍去)或 q =-2.故{ an }的公比为-2.
(2)若 a 1=1,求数列{ nan }的前 n 项和.
错位相减法求数列{an}的前 n 项和
若{ an }是公差为 d ( d ≠0)的等差数列,{ bn }是公比为 q ( q ≠1)的等比数 列,求数列{ an ·b n }的前 n 项和 Sn .
(1)在写出 Sn ,与 qSn ,的表达式时,应特别注意将两式“错位对齐”,以便 下一步准确写出 Sn - qSn ,;
(2)作差后,应注意减式中所剩各项的符号要变号.
4. 错位相减法万能公式
问题:在数列{ an }中,已知 .
(1)求{ an }的通项公式;
注:如果选择多个条件分别解答,按第一个解答计分.
教材延展 数列求和的其他方法
◉角度(一)分段求和法
在等差数列{ an }中, a 10=23, a 25=-22,
求:(1)数列{ an }前多少项和最大;
[解] 设等差数列{ an }的公差为 d ,
(2)数列{| an |}前 n 项和.
数列求和时,通项公式分段表示,求和时就要分两种情况求和,常 见项为绝对值的情况.
◉角度(二) 倒序相加法
(1)si n 21°+si n 22°+si n 23°+…+si n 288°+si n 289°= ;
(1)设 S =si n 21°+si n 22°+si n 23°+…+si n 288°+si n 289°,①
S =si n 289°+si n 288°+…+si n 23°+si n 22°+si n 21°,②
又∵si n x = cs (90°- x ),si n 2 x + cs 2 x =1,
2S =(si n 21°+ cs 21°)+(si n 22°+ cs 22°)+…+(si n 289°+ cs289°)=89,
(1)求数列{ an },{ bn }的通项公式;
若选③, bn =22-2lg2( an +1)=22-2lg2(2 n -1+1)=22-2 n .
1. 数列{ an }的前 n 项和为 Sn ,且 an =(-1) n (2 n -1),则 S 2 024= ( A )
an + an +1=(-1) n (2 n -1)+(-1) n +1(2 n +1)=(-1) n +1(2 n +1-2 n +1)=2×(-1) n +1,因而 S 2 024=( a 1+ a 2)+( a 3+ a 4)+…+( a 2 023+ a 2 024)=2×1 012=2 024.
a 2+ a 4+…+ a 100
=( a 1+ d )+( a 3+ d )+…+( a 99+ d )
=( a 1+ a 3+…+ a 99)+50 d
3. 设等差数列{ an }的前 n 项和为 Sn ,且满足 S 2 023>0, S 2 024<0, 对任意正整数 n ,都有| an |≥| ak |,则 k 的值为( C )
5. (多选)已知数列{ an }的首项为4,且满足2( n +1) an - nan +1=0( n ∈N*),则( BD )
6. (多选)(2024·山东济宁期末)若 Sn 为数列{ an }的前 n 项和,且 Sn = 2 an +1,则下列说法正确的是( AC )
因为 Sn 为数列{ an }的前 n 项和,且 Sn =2 an +1,所以 a 1= S 1=2 a 1+1,所以 a 1=-1.当 n ≥2时, an = Sn - Sn -1=2 an -2 an -1,即 an =2 an -1,所以数列{ an }是以-1为首项,2为公比的等比数列,故C正确; a 5=-1×24=-16,故A正确; Sn =2 an +1=-2 n +1,所以 S 5=-25+1 =-31,故B错误;因为 S 1+1=0,所以数列{ Sn +1}不是等比数列, 故D错误.
所以当 n 为奇数时, an =- n -1,所以B正确;
∵ S 2=3 a 1=3,∴ a 1=1, a 2=2.
S 7=1+2+2+4+4+8+8=29, S 8= S 7+ a 8=29+16=45>30,
∴ m 的最小值是8.
S 2 023= a 1+( a 2+ a 3)+( a 4+ a 5)+…+( a 2 022+ a 2 023),
∴ S 2 023=1+(2-1 009)+(4-1 009)+…+(2 022-1 009)
=1+(2+4+6+…+2 022)-1 009×1 011
又 a 1- a 2=0, a 1=3,所以 a 2=3,
又 a 1+ a 2- a 3=0,所以 a 3=6,
所以数列{ an }从第2项起构成以3为首项,以2为公比的等比数列,
所以数列{ an }的前2 024项和
所以数列{ Sn }是首项为 S 1= a 1=3,公比为2的等比数列,
所以 Sn =3×2 n -1,所以 S 2 024=3×22 023.
法二:设数列{ an }的前 n 项和为 Sn ,
14. (多选)如图的形状出现在南宋数学家杨辉所著的《详解九章算 法·商功》中,后人称为“三角垛”.“三角垛”从上往下数最上层有1个 球,第二层有3个球,第三层有6个球,….设第 n 层有 an 个球,从上往下 n 层球的总数为 Sn ,则( BCD )
a 2- a 1=2,
a 3- a 2=3,
an - an -1= n ,
所以 S 5= a 1+ a 2+ a 3+ a 4+ a 5=1+3+6+10+15=35,故选项 B正确;
由递推关系可知: an +1- an = n +1,故选项A不正确;
解得 k =4或 k =8(舍去),所以A选项正确. S 6=62-12×6=-36, a 7=(-3)0-1=0, a 8=(-3)1-1=-4, S 8=-36+0+(-4)=-40, 所以B选项错误. a 9=(-3)2-1=8, S 9=-40+8=-32,所以C选项正确. a 10=(-3)3-1=-28, a 11=(-3)4-1=80, a 12=(-3)5-1=-244,所以 S 12=-32-28+80-244=-224,所以D选项错误.
当 n =1时, a 1=2也符合上式,
∴ an =2 n ( n ∈N*),
∴2 an =2( n +1),∴ an = n +1.
2025版《优化探究》高考数学总复习人教版数学配套专题课件: 这是一份2025版《优化探究》高考数学总复习人教版数学配套专题课件,文件包含第四章三角函数docx、第二章函数的概念性质与基本初等函数docx、第七章立体几何与空间向量docx、第三章三角函数docx、第十章计算原理概率随机变量及其分布docx、第九章圆锥曲线的方程docx、第一章集合常用逻辑用语与不等式docx、第六章数列docx、第十一章统计与成对数据的分析docx、第五章平面向量与复数docx、第八章直线和圆的方程docx等11份课件配套教学资源,其中PPT共0页, 欢迎下载使用。
《高考总复习》数学 第七章 第8讲 轨迹与方程[配套课件]: 这是一份《高考总复习》数学 第七章 第8讲 轨迹与方程[配套课件],共42页。
《高考总复习》数学 第七章 第3讲 圆的方程[配套课件]: 这是一份《高考总复习》数学 第七章 第3讲 圆的方程[配套课件],共42页。