]福建省长泰一中高考数学一轮复习《不等式证明》(一)学案
1.比较法是证明不等式的一个最基本的方法,分比差、比商两种形式.
(1)作差比较法,它的依据是:
它的基本步骤:作差——变形——判断,差的变形的主要方法有配方法,分解因式法,分子有理化等.
(2) 作商比较法,它的依据是:若
>0,
>0,则
例1. 已知
,求证:
证法1:
=
=
=
∵
>0,
>0,
∴
即
证法2:
=1+
∴
故原命
成立,证毕.
变式训练1:已知a、b、x、y∈R+且
>
,x>y.
求证:
>
.
解:证法一:(作差比较法)
∵
-
=
,
又
>
且a、b∈R+,
∴b>a>0.又x>y>0,∴bx>ay.
∴
>0,即
>
.
证法二:(分析法)
∵x、y、a、b∈R+,∴要证
>
,
只需证明x(y+b)>y(x+a),即证xb>ya.
由
>
>0,∴b>a>0. 又x>y>0,知xb>ya显然成立.故原不等式成立.
例2. 已知a、b∈R+,求证:
∴
变式训练3:若
为△ABC的三条边,且
,则( )
A.
B.
C.
D.
:D.解析:
,
又∵
∴
。
例4. 设二次函数
,方程
的两个根
、
满足
.
(1) 当x∈(0,x1)时,证明:x
规范.
4.分析法和综合法是对立统一的两个方法.在不等式的证明中,我们常用分析法探索证明的途径后,用综合法的形式写出证明过程.这种先分析后综合的思路具有一般性,是解决数学问题的一种重要数学思想.
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