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Algebra Difficulty 6.6 National olympiad Prove it

Example 5 Let real numbers xix_{i} satisfy xi<1(i=1,2,,n),n2\left|x_{i}\right|<1(i=1,2, \cdots, n), n \geqslant 2, prove:
i=1n11xinn1i=1nxi\sum_{i=1}^{n} \frac{1}{1-\left|x_{i}\right|^{n}} \geqslant \frac{n}{1-\prod_{i=1}^{n} x_{i}}

Solution

Prove that by the Cauchy-Schwarz inequality, we have
i=1n11xini=1n(1xin)n2\sum_{i=1}^{n} \frac{1}{1-\left|x_{i}\right|^{n}} \cdot \sum_{i=1}^{n}\left(1-\left|x_{i}\right|^{n}\right) \geqslant n^{2} \text {. }

Therefore, to prove the original inequality, it suffices to prove

That is, to prove \square \square
n2i=1n(1xin)n1i=1nxinni=1nxii=1n(1xin)i=1nxinni=1nxi\begin{array}{c} \frac{n^{2}}{\sum_{i=1}^{n}\left(1-\left|x_{i}\right|^{n}\right)} \geqslant \frac{n}{1-\prod_{i=1}^{n} x_{i}} \\ n-n \prod_{i=1}^{n} x_{i} \geqslant \sum_{i=1}^{n}\left(1-\left|x_{i}\right|^{n}\right) \\ \sum_{i=1}^{n}\left|x_{i}\right|^{n} \geqslant n \prod_{i=1}^{n} x_{i} \end{array}

That is, \square
By the AM-GM inequality, the above inequality holds, hence the original proposition is true.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.