第10招:借花献佛-放缩法证明不等式
第10招:借花献佛 - 放缩法证明不等式
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在证明不等式的过程中,往往需要将代数式的某些部分适当地放大或缩小,从而得到相应的不等式,以达到解题的目的,这种解决问题的方法就称为放缩法.
放缩法的一般思路为:要让不等式
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成立,有时可以将它的一边放大或缩小,寻找一个中间量,例如:将
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放大成
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,即
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,再证
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即可达到证明的目的. 但在使用放缩法解题时的难点在于,要注意把握“放”和“缩”的度。
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1.放缩法是中学数学的常用解题技巧之一,特别适用于思维难度大、构造性强的题目,能全面而综合地考察学生的潜能和后续学习能力. 由于放缩的尺度不好把握,因而放缩法是一种较难的解题技巧,在学习过程中要善于总结,积累一些常用的函数不等式和解题模式.
2.常见函数放缩:
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常见变式为:
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常见变式为:
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3.其他常见放缩
分式放缩:
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数列放缩:
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(2017课标Ⅲ理21)已知函数
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.
(1)若
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,求
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的值;
(2)设
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为整数,且对于任意正整数
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,
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【答案】见解析
【解析】(1)
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的定义域为
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.
① 当
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时,因为
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,不符合题意;
② 当
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时,由
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知
当
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时,
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;当
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时,
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.
所以函数
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在
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单调递减,在
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单调递增,故
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是函数
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在
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上的唯一最小值点,由于
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,所以当且仅当
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时,
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.
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(2)由(1)知,当
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时,
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令
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,得
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,从而
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所以
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而
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,且
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为整数,所以
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的最小值为
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.
1.已知函数
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.
(1)若
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,求
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的取值范围;
(2)设
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,证明不等式
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.
2.已知函数
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.
(1)若
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在
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上单调递增,求
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的值;
(2)证明:对任意
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,都有
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.
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