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Algebra Difficulty 7.1 National olympiad, round 2 Prove it

\square Example 4 Given 5n5 n real numbers ri,si,ti,ui,vir_{i}, s_{i}, t_{i}, u_{i}, v_{i} all greater than 1(1in)1(1 \leqslant i \leqslant n), let
R=(1ni=1nri),S=(1ni=1nsi),T=(1ni=1nti),U=(1ni=1nui),V=(1ni=1nvi), prove that: i=1n(risitiuivi+1risitiuivi1)(RSTUV+1RSTUV1)n.\begin{array}{l} R=\left(\frac{1}{n} \sum_{i=1}^{n} r_{i}\right), S=\left(\frac{1}{n} \sum_{i=1}^{n} s_{i}\right), T=\left(\frac{1}{n} \sum_{i=1}^{n} t_{i}\right), U=\left(\frac{1}{n} \sum_{i=1}^{n} u_{i}\right), \\ V=\left(\frac{1}{n} \sum_{i=1}^{n} v_{i}\right), \text { prove that: } \prod_{i=1}^{n}\left(\frac{r_{i} s_{i} t_{i} u_{i} v_{i}+1}{r_{i} s_{i} t_{i} u_{i} v_{i}-1}\right) \geqslant\left(\frac{R S T U V+1}{R S T U V-1}\right)^{n} . \end{array}
(1994 Chinese National Training Team Problem)

Solution

Proof: Let x1,x2,,xn(1,+)x_{1}, x_{2}, \cdots, x_{n} \in(1,+\infty), then by the generalized Cauchy inequality we have
(1+x1)(1+x2)(1+xn)(1+x1x2xnn)n\left(1+x_{1}\right)\left(1+x_{2}\right) \cdots\left(1+x_{n}\right) \geqslant\left(1+\sqrt[n]{x_{1} x_{2} \cdots x_{n}}\right)^{n}

i.e., (1+x1)(1+x2)(1+xn)n1+x1x2xnn\sqrt[n]{\left(1+x_{1}\right)\left(1+x_{2}\right) \cdots\left(1+x_{n}\right)} \geqslant 1+\sqrt[n]{x_{1} x_{2} \cdots x_{n}}.
In (1), replace xix_{i} with xi1x_{i}-1 to get
(x11)(x21)(xn1)nx1x2xnn1\sqrt[n]{\left(x_{1}-1\right)\left(x_{2}-1\right) \cdots\left(x_{n}-1\right)} \leqslant \sqrt[n]{x_{1} x_{2} \cdots x_{n}}-1
(1) ÷\div (2) gives
(x1+1)(x2+1)(xn+1)(x11)(x21)(xn1)nx1x2xnn+1x1x2xnn1\sqrt[n]{\frac{\left(x_{1}+1\right)\left(x_{2}+1\right) \cdots\left(x_{n}+1\right)}{\left(x_{1}-1\right)\left(x_{2}-1\right) \cdots\left(x_{n}-1\right)}} \geqslant \frac{\sqrt[n]{x_{1} x_{2} \cdots x_{n}}+1}{\sqrt[n]{x_{1} x_{2} \cdots x_{n}}-1}

Take xi=risitiuivi(i=1,2,,n)x_{i}=r_{i} s_{i} t_{i} u_{i} v_{i}(i=1,2, \cdots, n) to get
i=1nrisitiuivi+1risitiuivi1(i=1nrisitiuivi+1i=1nrisitiuivi1)\prod_{i=1}^{n} \frac{r_{i} s_{i} t_{i} u_{i} v_{i}+1}{r_{i} s_{i} t_{i} u_{i} v_{i}-1} \geqslant\left(\frac{\sqrt{\prod_{i=1}^{n} r_{i} s_{i} t_{i} u_{i} v_{i}}+1}{\sqrt{\prod_{i=1}^{n} r_{i} s_{i} t_{i} u_{i} v_{i}}-1}\right)

By the AM-GM inequality, we have
i=1nrin1ni=1nri=R\sqrt[n]{\prod_{i=1}^{n} r_{i}} \leqslant \frac{1}{n} \sum_{i=1}^{n} r_{i}=R

Similarly, i=1nsinS,i=1ntinT,i=1nuinU,i=1nvinV\sqrt[n]{\prod_{i=1}^{n} s_{i}} \leqslant S, \sqrt[n]{\prod_{i=1}^{n} t_{i}} \leqslant T, \sqrt[n]{\prod_{i=1}^{n} u_{i}} \leqslant U, \sqrt[n]{\prod_{i=1}^{n} v_{i}} \leqslant V,
so
i=1nrisitiuivinRSTUV\sqrt[n]{\prod_{i=1}^{n} r_{i} s_{i} t_{i} u_{i} v_{i}} \leqslant R S T U V

Since y=x+1x1y=\frac{x+1}{x-1} is a decreasing function on (1,+)(1,+\infty), we have
i=1nrisitiuivi+1i=1nrisitiuivi1RSTUV+1RSTUV1\frac{\sqrt{\prod_{i=1}^{n} r_{i} s_{i} t_{i} u_{i} v_{i}}+1}{\sqrt{\prod_{i=1}^{n} r_{i} s_{i} t_{i} u_{i} v_{i}}-1} \geqslant \frac{R S T U V+1}{R S T U V-1}

By (3) and (4), the original inequality holds.

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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.