基本不等式知识点总结
向量不等式:
word/media/image1_1.png
【注意】:word/media/image2_1.png同向或有word/media/image3_1.pngword/media/image4_1.pngword/media/image5_1.pngword/media/image6_1.pngword/media/image7_1.png;
word/media/image2_1.png反向或有word/media/image3_1.pngword/media/image4_1.pngword/media/image8_1.pngword/media/image9_1.pngword/media/image10_1.png;
word/media/image2_1.png不共线word/media/image4_1.png.(这些和实数集中类似)
代数不等式:
word/media/image12_1.png同号或有word/media/image13_1.pngword/media/image14_1.png;
word/media/image12_1.png异号或有word/media/image15_1.pngword/media/image16_1.png.
绝对值不等式: word/media/image17_1.png
word/media/image18_1.png
双向不等式:word/media/image19_1.png
(左边当word/media/image20_1.png时取得等号,右边当word/media/image21_1.png时取得等号.)
放缩不等式:
①word/media/image22_1.png,则word/media/image23_1.png.
【说明】:word/media/image24_1.png(word/media/image25_1.png,糖水的浓度问题).
【拓展】:word/media/image26_1.pngword/media/image27_1.png.
②word/media/image28_1.png,word/media/image29_1.png,则word/media/image30_1.png;
③word/media/image31_1.png,;
④word/media/image33_1.png,word/media/image34_1.png.
⑤word/media/image35_1.pngword/media/image36_1.png,word/media/image37_1.pngword/media/image38_1.png.
word/media/image39.gif函数fbea0d4ccf5b7b7b8b4b684cfb9664d6.png
(1)函数28380e1704dd73d28243d05cfcbb6daf.png
(2)函数28380e1704dd73d28243d05cfcbb6daf.png
①值域:f8d6c5b9ed85e736dc5f024df21d1b8d.png
②单调递增区间:288fc2e78996433cb1557402fb15bae0.png
基本不等式知识点总结
重要不等式
1、和积不等式:word/media/image48_1.pngword/media/image49_1.pngword/media/image50_1.png(当且仅当word/media/image51_1.png时取到“word/media/image52_1.png”).
【变形】:①word/media/image53_1.png(当a = b时,word/media/image54_1.png)
【注意】: word/media/image55_1.png,
2、均值不等式:
两个正数word/media/image57_1.png的调和平均数、几何平均数、算术平均数、均方根之间的关系,即“平方平均word/media/image58_1.png算术平均word/media/image58_1.png几何平均word/media/image58_1.png调和平均”
word/media/image59_1.png
*.若887fb68a10cbd4369b27c90bee0334d8.png
若97fdf90850f660f05349f4ad145b62dc.png
若7f36509f20b76eba66f86f3316fd7d8e.png
*.若6f5e3a8e1c8466c3503af36bfbff2002.png
若e660b4637c687b54a163ef972eabbe41.png
3、含立方的几个重要不等式(a、b、c为正数):
word/media/image73_1.png(,word/media/image75_1.png);
word/media/image76_1.png word/media/image77_1.pngword/media/image78_1.pngword/media/image79_1.png
*不等式的变形在证明过程中或求最值时,有广泛应用,如:当6f5e3a8e1c8466c3503af36bfbff2002.png
*2daa2e159899452ff433938173c2355a.png
八种变式: ①21d473d775d5f91e234ee869162ef33c.png
④5b2d4ca1e75eb5efbceaa9cff0a193e9.png
上述八个不等式中等号成立的条件都是“7acaac15494e6820b1ed6d8b539af089.png
最值定理
(积定和最小)
①word/media/image96_1.png,若积word/media/image97_1.png,则当word/media/image98_1.png时和word/media/image99_1.png有最小值;
(和定积最大)
②word/media/image101_1.png,若和word/media/image102_1.png,则当word/media/image98_1.png是积word/media/image103_1.png有最大值word/media/image104_1.png.
【推广】:已知word/media/image105_1.png,则有word/media/image106_1.png.
(1)若积word/media/image107_1.png是定值,则当word/media/image108_1.png最大时,word/media/image109_1.png最大;当word/media/image108_1.png最小时,最小.
(2)若和word/media/image111_1.png是定值,则当word/media/image108_1.png最大时,word/media/image112_1.png最小;当word/media/image108_1.png最小时,word/media/image113_1.png最大.
③已知word/media/image114_1.png,若word/media/image115_1.png,则有则word/media/image116.gif的最小值为:word/media/image117_1.png
④已知word/media/image116.gif,若word/media/image116.gif则word/media/image116.gif和word/media/image116.gif的最小值为:
①word/media/image116.gif.
②word/media/image116.gif
应用基本不等式求最值的“八种变形技巧”:
word/media/image118.gif凑系数(乘、除变量系数).例1.当 word/media/image119_1.png时,求函的数word/media/image120_1.png最大值.
word/media/image121.gif凑项(加、减常数项):例2.已知word/media/image122_1.png ,求函数word/media/image123_1.png的最大值.
word/media/image124.gif调整分子:例3.求函数word/media/image125_1.png的值域;
word/media/image126.gif变用公式:基本不等式word/media/image127_1.png有几个常用变形,word/media/image128_1.png,word/media/image129_1.png不易想到,应重视;
例4.求函数word/media/image130_1.png的最大值;
word/media/image131.gif连用公式:例5.已知word/media/image132_1.png,求word/media/image133_1.png的最小值;
word/media/image134.gif对数变换:例6.已知word/media/image135_1.png,且word/media/image136_1.png,求word/media/image137_1.png的最大值;
word/media/image138.gif三角变换:例7.已知word/media/image139_1.png,且word/media/image140_1.png,求word/media/image141_1.png的最大值;
word/media/image142.gif常数代换(逆用条件):例8.已知word/media/image143_1.png,且word/media/image144_1.png,求word/media/image145_1.png的最小值.
“单调性”补了“基本不等式”的漏洞:
word/media/image118.gif平方和为定值
若word/media/image146_1.png(word/media/image147_1.png为定值,word/media/image148_1.png),可设word/media/image149_1.png,其中word/media/image150_1.png.
①word/media/image151_1.png在上是增函数,在word/media/image153_1.png上是减函数;
②word/media/image154_1.png在word/media/image155_1.png上是增函数,在上是减函数;
③word/media/image157_1.png.令word/media/image158_1.png,其中word/media/image159_1.png.由word/media/image160_1.png,得word/media/image161_1.png,从而word/media/image162_1.png在word/media/image163_1.png上是减函数.
word/media/image121.gif和为定值
若word/media/image164_1.png(word/media/image165_1.png为定值,word/media/image166_1.png),则word/media/image167_1.png
①word/media/image168_1.png在word/media/image169_1.png上是增函数,在word/media/image170_1.png上是减函数;
②word/media/image171_1.png.当word/media/image172_1.png时,在word/media/image173_1.png上是减函数,在word/media/image174_1.png上是增函数;当word/media/image175_1.png时,在word/media/image176_1.png上是减函数,在上是增函数.
③word/media/image178_1.png在word/media/image169_1.png上是减函数,在word/media/image170_1.png上是增函数;
word/media/image124.gif积为定值
若word/media/image179_1.png(word/media/image180_1.png为定值,word/media/image181_1.png),则word/media/image182_1.png
①word/media/image183_1.png.当word/media/image184_1.png时,在word/media/image185_1.png上是减函数,在word/media/image186_1.png上是增函数;当word/media/image187_1.png时,在word/media/image188_1.png上是增函数;
②word/media/image189_1.png.当word/media/image184_1.png时,在word/media/image185_1.png上是减函数,在word/media/image186_1.png上是增函数;当word/media/image187_1.png时,在word/media/image188_1.png上是减函数;
③word/media/image190_1.png在word/media/image191_1.png上是减函数,在word/media/image192_1.png上是增函数.
word/media/image126.gif倒数和为定值
若word/media/image193_1.png(word/media/image194_1.png为定值,word/media/image195_1.png),则word/media/image182_1.png成等差数列且均不为零,可设公差为word/media/image196_1.png,其中word/media/image197_1.png,则得word/media/image199_1.png.
①word/media/image200_1.png.当word/media/image201_1.png时,在word/media/image202_1.png上是减函数,在word/media/image203_1.png上是增函数;当word/media/image204_1.png时,在word/media/image205_1.png上是增函数,在word/media/image206_1.png上减函数;
②word/media/image207_1.png.当word/media/image201_1.png时,在word/media/image202_1.png上是减函数,在word/media/image203_1.png上是增函数;当word/media/image204_1.png时,在word/media/image205_1.png上是减函数,在word/media/image206_1.png上是增函数;
③word/media/image208_1.png.令word/media/image209_1.png,其中word/media/image210_1.png且word/media/image211_1.png,从而word/media/image212_1.png在word/media/image213_1.png上是增函数,在word/media/image214_1.png上是减函数.
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