专题 11 不等式选讲
2021高考数学(理)高频考点、热点题型归类强化
【高频考点及备考策略】
本部分内容在备考时应注意以下几个方面:
不等式选讲也是高考必考内容,重点考查绝对值不等式的解法、不等式的证明及求参数取值范围问题,题型多为解答题,难度为中档.
考向预测:
(1)绝对值不等式的解法;
(2)不等式的证明;
(3)绝对值不等式恒成立(存在)问题;
1.绝对值不等式
定理1:如果a,b是实数,则|a+b|≤|a|+|b|,当且仅当ab≥0时,等号成立.
定理2:如果a,b,c是实数,那么|a-c|≤|a-b|+|b-c|,当且仅当(a-b)(b-c)≥0时,等号成立.
2.绝对值不等式的解法
(1)|ax+b|≤c(c>0)和|ax+b|≥c(c>0)型不等式的解法
①|ax+b|≤c(c>0)⇔-c≤ax+b≤c.
②|ax+b|≥c(c>0)⇔ax+b≥c或ax+b≤-c.
(2)|x-a|+|x-b|≥c(c>0)和|x-a|+|x-b|≤c(c>0)型不等式的解法
①利用绝对值不等式几何意义求解,体现数形结合思想.
②利用“零点分段法”求解,体现分类讨论思想.
③通过构建函数,利用函数图象求解,体现函数与方程思想.
3.证明不等式的基本方法
(1)比较法;(2))综合法;(3)分析法;(4)反证法;(5)放缩法.
4.二维形式的柯西不等式
若a,b,c,d∈R,则(a2+b2)(c2+d2)≥(ac+bd)2,当且仅当ad=bc时,等号成立.
【易错警示】
1.应用绝对值不等式性质求函数的最值时,一定要注意等号成立的条件.特别是多次使用不等式时,必须使等号同时成立.
2.利用基本不等式证明要注意“一正、二定、三相等”三个条件同时成立,缺一不可.
3.在去掉绝对值符号进行分类时要做到不重不漏.
(2020新课标Ⅰ卷·理科T23)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/50880e1d22a971fea0d681f0048effff.png)
(1)画出
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/51c530f4b7491b95acb8ce3eef9aa09d.png)
![](https://img.xkw.com/dksih/QBM/editorImg/2022/11/29/09a25c44-6537-41f3-9302-7e9a5624be09.png?resizew=223)
(2)求不等式
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/bc3e238b1bc55e6240949c28d9561a94.png)
【知识点】 图象法解绝对值不等式解读
(2020新课标Ⅱ卷·理科T23)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/54eb6ad7c908f94d867f3230a07de08c.png)
(1)当
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/8e258ab9e600435b37465092243d99f6.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/423314079e772f46d346caafd68a0631.png)
(2)若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/423314079e772f46d346caafd68a0631.png)
【知识点】 分类讨论解绝对值不等式解读
(2020新课标Ⅲ卷·理科T23)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a02d44492b51b0e08208fdc0d1707025.png)
(1)证明:ab+bc+ca<0;
(2)用max{a,b,c}表示a,b,c中的最大值,证明:max{a,b,c}≥
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5263a1d78ec9ea895f3b7781fb36e008.png)
【知识点】 基本(均值)不等式的应用解读
(2020江苏卷·T23)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c4166972dec0aa3e8694a44eeb941a08.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c77fcd9e188b6df52ca0c8a04d10da02.png)
【知识点】 几何意义解绝对值不等式解读
考点 一 绝对值不等式的解法
(1)当a=5时,解关于x的不等式f(x)>9;
(2)设关于x的不等式f(x)≤|x-4|的解集为A,若B={x∈R||2x-1|≤3},当A∪B=A时,求实数a的取值范围.
【知识点】 根据并集结果求集合或参数解读 分类讨论解绝对值不等式解读
【备考策略】
解决含绝对值不等式问题
解形如|x-a|+|x-b|≥c(或≤c)的不等式主要有两种方法:
①分段讨论法:将数轴分为(-∞,a],(a,b],(b,+∞)(此处设a<b)三个部分,在每部分区间内去掉绝对值符号并分别列出对应的不等式求解,然后取各个不等式解集的并集;
②图像法:作出函数y1=|x-a|+|x-b|和y2=c的图像,结合图像求解.
【类比演练】
已知函数
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/58ebdc4d97e166405362b400d54f0903.png)
(1)解关于
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/81dea63b8ce3e51adf66cf7b9982a248.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2608a57caffde627dbf140ca22a2ff8a.png)
(2)设
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/94440d3e4c073f94f2b266ff99d50e74.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/81dea63b8ce3e51adf66cf7b9982a248.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f1df29f303dda1c198a21382a1bbbf6a.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0a6936d370d6a238a608ca56f87198de.png)
【知识点】 含绝对值不等式的解法 解含参数的绝对值不等式解读
考点 二 不等式的证明
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/94440d3e4c073f94f2b266ff99d50e74.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/67ca5fd57c2c2fcc3c7a574fdd1467d9.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/da7dec5f5ef4a53175966c1704ad8a15.png)
证明:(1)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5f1a2cbcc1fb41a01668f1808267df4d.png)
(2)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/656cacf9b32ce8f718dcb50bc8994593.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/04f678dde8a2f44b8eae985b11bf4b50.png)
【知识点】 由基本不等式证明不等关系解读 反证法证明解读
【备考策略】
本题主要考查了不等式的证明与反证法等知识点,属于中档题,第一小问需将条件中的式子作等价变形,再利用基本不等式即可求解;第二小问从问题不可能同时成立,可以考虑采用反证法证明,否定结论,从而推出矛盾,反证法作为一个相对冷门的数学方法,在后续复习时亦应予以关注.
【类比演练】
(1)求M的值;
(2)正数a,b,c满足a+2b+c=M,求证:
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e9150192ba96228d43023376904f9fa7.png)
考点 二 绝对值不等式恒成立(存在)问题
【典例】
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/002e83379e9cc15656508e7455bc9a0f.png)
(1)若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6b108ab31cc093f03cf48ad65429889e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/369100ccd44feaa77e5f119ea949a879.png)
(2)如果关于
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/81dea63b8ce3e51adf66cf7b9982a248.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2e2405c4822bceae1cf191edb502d3b0.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0a6936d370d6a238a608ca56f87198de.png)
【备考策略】
1.求含绝对值号函数的值的两种方法
(1)利用|a|-|b|≤|a±b|≤|a|+|b|求解.
(2)将函数化为分段函数,数形结合求解.
2.恒成立(存在)问题的等价转化
f(x)≥M | f(x)≤M | |
任意x恒成立⇔ | f(x)min≥M | f(x)max≤M |
存在x成立⇔ | f(x)max≥M | f(x)min≤M |
【类比演练】
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/aa02b8c401af4fd18b6b18059a0fbe37.png)
(1)若
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/1485a4756c56f1126b9825d5019d544c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/26e7d4694604abe0ebc36818ee192a4a.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/294f5ba74cdf695fc9a8a8e52f421328.png)
(2)求不等式
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2ca5f62e36c402a39ad53d1095d7028a.png)
【知识点】 求绝对值不等式中参数值或范围解读
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/83885a8e289d2f6d47ae4c6d65472761.png)
(1)求不等式
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ab989f85a681b41c29465d4be74b789f.png)
(2)若不等式
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5645aeacf7a83dc2ed195e9cbfdb7b9d.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/4aa0df7f1e45f9de29e802c7f19a4f64.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/294f5ba74cdf695fc9a8a8e52f421328.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e060689d5e2e31792be895bc6df7e2c6.png)
(Ⅰ)当
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e65397f11ea8af736f38debadf420c4a.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/bca88b72ac8dc9c7c137af932de90bc7.png)
(Ⅱ)当
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ff179adf5fcfe1a8a1298796cb0a97ff.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/030805eebb5ad0a065f93bd6f652f687.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0a6936d370d6a238a608ca56f87198de.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2a73c6f617b4095ce2d4de4be860ec78.png)
(1)若不等式
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/3601b55635207a5558f3114f95ba6745.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/294f5ba74cdf695fc9a8a8e52f421328.png)
(2)当
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ff42751c7c8e5354c22235b651176650.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/16a78988278258e2024c706b55bb5067.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/0a6936d370d6a238a608ca56f87198de.png)
【知识点】 绝对值三角不等式解读 解含参数的绝对值不等式解读
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/30fd9007bd0b6a53aca849f0a4e17fad.png)
(1)求不等式
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/a4d0cd47609b9d1865dfff4979161cf5.png)
(2)已知
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7b80dc1048e670f72c0c9fc7b10203a2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/4fe7d5809da02c15a43a0e9a898b9086.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/9677dc5143159dd751428d6db19cc594.png)