裂项相消法
前言概述
用裂项相消法可以求数列的通项公式\(a_n\),也可以求数列的前\(n\)项的和\(S_n\)。
常用公式
常用式:\(\cfrac{1}{n(n+1)}=\cfrac{1}{n}-\cfrac{1}{n+1}\);推广式:\(\cfrac{1}{n(n+k)}=\cfrac{1}{k}(\cfrac{1}{n}-\cfrac{1}{n+k})\);
常用式:\(\cfrac{1}{\sqrt{n+1}+\sqrt{n}}=\sqrt{n+1}-\sqrt{n}\);推广式:\(\cfrac{1}{\sqrt{n+k}+\sqrt{n}}=\cfrac{1}{k}(\sqrt{n+k}-\sqrt{n})\);
不常用:\(\cfrac{1}{\sqrt{2n-1}+\sqrt{2n+1}}=\cfrac{1}{2}(\sqrt{2n+1}-\sqrt{2n-1})\);
常用式:\(\cfrac{1}{4n^2-1}=\cfrac{1}{(2n-1)(2n+1)}=\cfrac{1}{2}(\cfrac{1}{2n-1}-\cfrac{1}{2n+1})\);
组合用:\(\cfrac{n^2}{4n^2-1}=\cfrac{1}{4}\times\cfrac{4n^2-1+1}{4n^2-1}=\cfrac{1}{4}(1+\cfrac{1}{4n^2-1})=\cfrac{1}{4}+\cfrac{1}{8}(\cfrac{1}{2n-1}-\cfrac{1}{2n+1})\);
常用式:\(ln(1+\cfrac{1}{n})=ln\cfrac{n+1}{n}=ln(n+1)-lnn\);
不常用:\(\cfrac{a_{n+1}}{S_n\cdot S_{n+1}}=\cfrac{S_{n+1}-S_{n}}{S_n\cdot S_{n+1}}=\cfrac{1}{S_n}-\cfrac{1}{S_{n+1}}\)
不常用:\(\cfrac{2^n}{(2^n-1)(2^{n+1}-1)}=\cfrac{1}{2^n-1}-\cfrac{1}{2^{n+1}-1}\)
不常用:\(\cfrac{n+1}{(n+2)^2\cdot n^2}=\cfrac{1}{4}[\cfrac{1}{n^2}-\cfrac{1}{(n+2)^2}]\)
不常用:\(\cfrac{1}{n(n+1)(n+2)}=\cfrac{1}{2}[\cfrac{1}{n(n+1)}-\cfrac{1}{(n+1)(n+2)}]\),启迪思维的变形,两项到三项
记忆方法
【案例1】\(\cfrac{2}{(n-1)(n+1)}=2\cdot \cfrac{1}{(n-1)(n+1)}=2\cdot \Box (\cfrac{1}{n-1}-\cfrac{1}{n+1})\),
那么小括号前面的系数到底该是多少才能使得原式保持恒等变形呢?
我们只需要做通分的工作,将
\(\cfrac{1}{n-1}-\cfrac{1}{n+1}=\cfrac{(n+1)-(n-1)}{(n-1)(n+1)}=\cfrac{2}{(n-1)(n+1)}\)
故\(\cfrac{1}{(n-1)(n+1)}=\cfrac{1}{2}(\cfrac{1}{n-1}-\cfrac{1}{n+1})\),
故上述\(\Box\)位置应该为\(\cfrac{1}{2}\),
即\(\cfrac{2}{(n-1)(n+1)}=2\cdot \cfrac{1}{2} (\cfrac{1}{n-1}-\cfrac{1}{n+1})=\cfrac{1}{n-1}-\cfrac{1}{n+1}\),
【案例2】\((\sqrt{n+1}+\sqrt{n})(\sqrt{n+1}-\sqrt{n})=1\),故\(\cfrac{1}{\sqrt{n+1}+\sqrt{n}}=\sqrt{n+1}-\sqrt{n}\)
关联表示
①\(\cfrac{1}{n^2+2n}\);②\(b_n=\cfrac{1}{a_n\cdot a_{n+1}}\);③\(\cfrac{a_{n+1}}{S_n\cdot S_{n+1}}\);④\(c_n=\cfrac{1}{log_3{a_n}\cdot log_3{a_{n+1}}}\);
⑤\(\cfrac{1}{(n+2)^2-4}=\cfrac{1}{n^2+4n}=\cfrac{1}{n(n+4)}=\cfrac{1}{4}(\cfrac{1}{n}-\cfrac{1}{n+4})\);⑥ \(b_n=\cfrac{a_n}{(a_n+1)(a_{n+1}+1)}\);
⑦ 意想不到的一种:由 \(b_{n+1}b_n=2n-1\),得到\(\cfrac{1}{b_n}\)的裂项公式 [1]
书写模式
如数列\(a_n=\cfrac{1}{n(n+2)}\),求其前\(n\)项和\(S_n\)。
分析:先裂项得到,\(a_n=\cfrac{1}{2}(\cfrac{1}{n}-\cfrac{1}{n+2})\),
则\(S_n=a_1+a_2+a_3+\cdots+a_n\)
则往下的求和书写格式有以下两种:
第一种书写格式:横向消项,容易出错;
\(S_n=\cfrac{1}{2}[(1-\cfrac{1}{3})+(\cfrac{1}{2}-\cfrac{1}{4})+(\cfrac{1}{3}-\cfrac{1}{5})+\cdots+(\cfrac{1}{n-1}-\cfrac{1}{n+1})+(\cfrac{1}{n}-\cfrac{1}{n+2})]\)
\(=\cfrac{1}{2}(1+\cfrac{1}{2}-\cfrac{1}{n+1}-\cfrac{1}{n+2})=\cdots\)
第二种书写格式:纵向消项,不易出错,如图所示;
先得到如下的表达式,
\(S_n=\cfrac{1}{2}[(1-\cfrac{1}{3})+(\cfrac{1}{2}-\cfrac{1}{4})+(\cfrac{1}{3}-\cfrac{1}{5})+\cdots+(\cfrac{1}{n-1}-\cfrac{1}{n+1})+(\cfrac{1}{n}-\cfrac{1}{n+2})]\)
然后如图所示,将每一个小括号写成两列,
很明显可以斜向消项,第一列剩余前两项,第二列剩余后两项,故结果为
\(S_n=\cfrac{1}{2}(1+\cfrac{1}{2}-\cfrac{1}{n+1}-\cfrac{1}{n+2})=\cdots\)
再给一个练习题,通过此练习题,更能体会纵向书写的妙处。
\(a_n=\cfrac{1}{n(n+3)}\),求其前\(n\)项和\(S_n\)。
横向写法的好处:比如数列 \(b_1\),\(b_2-b_1\),\(b_3-b_2\),\(\cdots\),\(b_n-b_{n-1}\) 的前 \(n\) 项的和为 \(b_n\);
解释:\(b_1\)\(+\)\((b_2-b_1)\)\(+\)\((b_3-b_2)\)\(+\)\(\cdots\)\(+\)\((b_n-b_{n-1})=b_n\);
求通项公式
分析:两边同除以\(n(n+1)\),得到\(\cfrac{a_{n+1}}{n+1}=\cfrac{a_n}{n}+\cfrac{2}{n(n+1)}\),再用累加法,得到\(a_n=4n-2\);
典例剖析
问题:\(\cfrac{k\cdot 2^{k+1}}{(k+1)(k+2)}=\cfrac{2^{k+2}}{k+2}-\cfrac{2^{k+1}}{k+1}\)是如何变形得到的?
分析: 这样的变形是为了利用数列\(\{\cfrac{2^{k+1}}{k+1}\}\)完成消项。
\(\cfrac{k\cdot 2^{k+1}}{(k+1)(k+2)}\)
\(=2^{k+1}\cdot\cfrac{k}{(k+1)(k+2)}\)
\(=2^{k+1}\cdot \cfrac{2(k+1)-(k+2)}{(k+1)(k+2)}\)
\(=2^{k+1}\cdot (\cfrac{2}{k+2}-\cfrac{1}{k+1})\)
\(=\cfrac{2^{k+2}}{k+2}-\cfrac{2^{k+1}}{k+1}\)
分析:先求得\(a_n=2^{2n-1}\),则\(b_n=1-2n\),
且数列\(\{\cfrac{1}{b_n\cdot b_{n+1}}\}\)的通项公式为\(\cfrac{1}{b_n\cdot b_{n+1}}=\cfrac{1}{2}(\cfrac{1}{2n-1}-\cfrac{1}{2n+1})\),
故\(T_n=\cdots=\cfrac{n}{2n+1}\)。
分析:必须先能认出其通项公式\(a_n=\cfrac{1}{1+2+3+\cdots+n}\),
从而\(a_n=\cfrac{1}{\cfrac{n(n+1)}{2}}=\cfrac{2}{n(n+1)}=2(\cfrac{1}{n}-\cfrac{1}{n+1})\),故有
\(S_n=a_1+a_2+\cdots+a_n\)
\(=2[(1-\cfrac{1}{2})+(\cfrac{1}{2}-\cfrac{1}{3})+(\cfrac{1}{3}-\cfrac{1}{4})+\cdots+(\cfrac{1}{n}-\cfrac{1}{n+1})]\)
\(=2(1-\cfrac{1}{n+1})=\cfrac{2n}{n+1}\)。
分析:由\(a_1+2d=3\)和\(4a_1+6d=10\),容易计算出\(a_n=n\),故\(S_n=\cfrac{n(n+1)}{2}\),
则有\(\cfrac{1}{S_n}=\cfrac{2}{n(n+1)}=2(\cfrac{1}{n}-\cfrac{1}{n+1})\),
故\(\sum\limits_{k=1}^n {\cfrac{1}{S_k}}\)
\(=2[(1-\cfrac{1}{2})+(\cfrac{1}{2}-\cfrac{1}{3})+\cdots +(\cfrac{1}{n}-\cfrac{1}{n+1})]\)
\(=2(1-\cfrac{1}{n+1})=\cfrac{2n}{n+1}\)。
分析:由新定义可知,\(\cfrac{n}{a_1+a_2+\cdots+a_n}=\cfrac{1}{2n+1}\),
则由上式得到,\(S_n=n(2n+1)\),又由\(a_n\)与\(S_n\)的关系可知,
当\(n\geqslant 2\)时,\(S_{n-1}=(n-1)[2(n-1)+1]\),则\(a_n=S_n-S_{n-1}=4n-1\);
再验证\(n=1\)时,\(a_1=1(2\times 1+1)=3=4\times 1-1\),满足上式,
故\(a_n=4n-1(n\in N^*)\),
则结合题目可知,故\(b_n=\cfrac{a_n+1}{4}=\cfrac{4n-1+1}{4}=n\),
则\(\cfrac{1}{b_1b_2}+\cfrac{1}{b_2b_3}+\cfrac{1}{b_3b_4}+\cdots+\cfrac{1}{b_{10}b_{11}}\)
\(=[(1-\cfrac{1}{2})+(\cfrac{1}{2}-\cfrac{1}{3})+\cdots+(\cfrac{1}{10}-\cfrac{1}{11})]=\cfrac{10}{11}\),
故选\(C\)。
(1)求数列\(\{a_n\}\)的通项公式。
分析:本题是利用\(a_n\)和\(S_n\)的关系解题,或者是利用“退一法”解题。
由题目可知,\(n\ge 1\),\(a_1+3a_2+\cdots+(2n-1)a_n=2n①\)得到,
当\(n\ge 2\),\(a_1+3a_2+\cdots+(2n-3)a_{n-1}=2(n-1)②\)
两式相减得到
\(n\ge 2,(2n-1)a_n=2\),
从而得到\(a_n=\cfrac{2}{2n-1}(n\ge 2)\),
接下来验证\(n=1\)是否满足
当\(n=1\)时,\(a_1=2=\cfrac{2}{2\times 1-1}\),满足上式,
故数列\(\{a_n\}\)的通项公式为\(a_n=\cfrac{2}{2n-1}(n\in N^*)\).
(2)求数列\(\{\cfrac{a_n}{2n+1}\}\)的前\(n\)项和\(S_n\)。
分析:结合第一问,数列\(\cfrac{a_n}{2n+1}=\cfrac{2}{(2n-1)(2n+1)}=\cfrac{1}{2n-1}-\cfrac{1}{2n+1}\)
故数列的前\(n\)项和\(S_n=(\cfrac{1}{2\times1-1}-\cfrac{1}{2\times 1+1})+(\cfrac{1}{2\times 2 -1}-\cfrac{1}{2\times 2+1})+\cdots+(\cfrac{1}{2n-1}-\cfrac{1}{2n+1})\)
\(=1-\cfrac{1}{2n+1}=\cfrac{2n}{2n+1}\)。
分析:当\(n\geq 2\)时,\(a_{n}-a_{n-1}=n\),\(a_{n-1}-a_{n-2}=n-1\), \(\ldots\), \(a_{2}-a_{1}=2\),
并项相加,得: \(a_{n}-a_{1}=n+(n-1)+\ldots+3+2\),
则有\(a_{n}=1+2+3+\ldots+n=\frac{1}{2} n(n+1)(n\geqslant 2)\)
又当\(n=1\)时,\(a_1=\cfrac{1}{2}\times 1\times (1+1)=1\),也满足上式,
故数列\(\{a_n\}\)的通项公式为\(a_n=\cfrac{1}{2}n(n+1)(n\in N^*)\);
则由题目\(b_{n}=\cfrac{1}{a_{n+1}}+\cfrac{1}{a_{n+2}}+\cfrac{1}{a_{n+3}}+\ldots+\cfrac{1}{a_{2n}}\)
\(=\cfrac{2}{(n+1)(n+2)}+\cfrac{2}{(n+2)(n+3)}+\cdots+\cfrac{2}{2n(2n+1)}\)
\(=2(\cfrac{1}{n+1}-\cfrac{1}{n+2}+\cfrac{1}{n+2}-\cfrac{1}{n+3}+\cdots+\cfrac{1}{2n}-\cfrac{1}{2n+1})\)
\(=2(\cfrac{1}{n+1}-\cfrac{1}{2n+1})=\cfrac{2n}{2n^{2}+3n+1}=\cfrac{2}{2n+\cfrac{1}{n}+3}\),
令\(f(x)=2 x+\cfrac{1}{x}(x \geq 1)\),则\(f'(x)=2-\cfrac{1}{x^2}\),
当\(x\geqslant 1\)时,\(f'(x)>0\)恒成立,故\(f(x)\)在\([1,+\infty)\)上为增函数,
故当$ x=1$ 时,\(f(x)_{\min}=f(1)=3\),即当 \(n=1\) 时,\((b_{n})_{\max}=\cfrac{1}{3}\)
则须使 \(m^{2}-mt+\cfrac{1}{3}>(b_{n})_{max}=\cfrac{1}{3}\), 即 \(m^{2}-mt >0\) 对\(\forall m \in[1,2]\) 恒成立,
即\(t<m\) 的最小值,可得 \(t<1\),故实数 \(t\) 的取值范围为\((-\infty, 1)\).
说明:一轮的复习中,必须要掌握每一个单独的知识点:①累加法求通项公式;②裂项法求通项公式;③简单的恒成立命题求解思路;
二轮复习中,就可以将对以上单独的知识点的考查整合到一个题目中,综合考察,很显然考查的难度会随着加入的知识点的个数,逐渐加大。
分析:由\(\sqrt{a_1}+\sqrt{a_2}+\sqrt{a_3}+\cdots+\sqrt{a_n}=n^2\),
故当\(n\ge 2\)时,\(\sqrt{a_1}+\sqrt{a_2}+\sqrt{a_3}+\cdots+\sqrt{a_{n-1}}=(n-1)^2\),
两式相减,得到
当\(n\ge 2\)时,\(\sqrt{a_n}=n^2-(n-1)^2=2n-1\),即\(a_n=(2n-1)^2\),
验证\(n=1\)时,也满足上式。故通项公式为\(a_n=(2n-1)^2,n\in N^*\),
\(a_{n+1}=(2n+1)^2=4n^2+4n+1\),
则\(\cfrac{1}{a_{n+1}-1}=\cfrac{1}{4n(n+1)}=\cfrac{1}{4}(\cfrac{1}{n}-\cfrac{1}{n+1})\)
故\(T_n=\cfrac{1}{4}[(1-\cfrac{1}{2})+(\cfrac{1}{2}-\cfrac{1}{3})+\cdots+(\cfrac{1}{n}-\cfrac{1}{n+1})]\)
\(=\cfrac{1}{4}\cdot \cfrac{n}{n+1}=\cfrac{n}{4n+4}\)
对应练习
提示:\(d=2\),\(\cfrac{a_{n+1}}{S_n\cdot S_{n+1}}=\cfrac{S_{n+1}-S_{n}}{S_n\cdot S_{n+1}}=\cfrac{1}{S_n}-\cfrac{1}{S_{n+1}}\)
\(\cfrac{a_2}{S_1S_2}+\cfrac{a_3}{S_2S_3}+\cfrac{a_4}{S_3S_4}+\cdots+\cfrac{a_{n+1}}{S_nS_{n+1}}=1-\cfrac{1}{(n+1)^2}\);
提示:变形得到\((a_{n+2}-a_{n+1})-(a_{n+1}-a_n)=2\),即数列\(\{a_{n+1}-a_n\}\)为等差数列,再用累加法得到\(a_n=n(n+1)\),则\(\cfrac{1}{a_n}=\cfrac{1}{n}-\cfrac{1}{n+1}\),则\(\cfrac{1}{a_1}+\cfrac{1}{a_2}+\cdots +\cfrac{1}{a_{2017}}=1-\cfrac{1}{2018}\),
则\(2017(\cfrac{1}{a_1}+\cfrac{1}{a_2}+\cdots +\cfrac{1}{a_{2017}})=2017(1-\cfrac{1}{2018})=2017-\cfrac{2017}{2018}=2016+\cfrac{1}{2018}\),
故\([\cfrac{2017}{a_1}+\cfrac{2017}{a_2}+\cdots +\cfrac{2017}{a_{2017}}]=[2016+\cfrac{1}{2018} ]=2016\)。
由 \(b_{n+1}b_n=2n-1\),得到 \(b_nb_{n-1}=2n-3\),两式相减得到,
\(b_n(b_{n+1}-b_{n-1})=2\),故 \(\cfrac{2}{b_n}=b_{n+1}-b_{n-1}\),这样就得到了项的倒数的裂项公式; ↩︎