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Algebra Difficulty 7.2 National olympiad, round 2 Prove it

Example 18 For each positive integer nn, let pn=(1+1n)n,Pn=(1+1n)n+1,hn=2pnPnpn+Pnp_{n}=\left(1+\frac{1}{n}\right)^{n}, P_{n}=\left(1+\frac{1}{n}\right)^{n+1}, h_{n}=\frac{2 p_{n} P_{n}}{p_{n}+P_{n}}, prove: h1<h2<<hn<hn+1h_{1}<h_{2}<\cdots<h_{n}<h_{n+1}. (6th Annual Putnam Mathematical Competition)

Solution

It is easy to derive that hn=2(n+1)n+1nn(2n+1)h_{n}=\frac{2(n+1)^{n+1}}{n^{n}(2 n+1)}. Consider the function g(x)g(x) defined by:
g(x)=ln2+(x+1)ln(x+1)xlnxln(2x+1)g(x)=\ln 2+(x+1) \ln (x+1)-x \ln x-\ln (2 x+1)

Then, for 0<x<+0<x<+\infty, we have
g(x)=ln(x+1)lnx22x+1g(x)=1x+11x+4(2x+1)2=1x(x+1)(2x+1)2<0\begin{array}{c} g^{\prime}(x)=\ln (x+1)-\ln x-\frac{2}{2 x+1} \\ g^{\prime \prime}(x)=\frac{1}{x+1}-\frac{1}{x}+\frac{4}{(2 x+1)^{2}}=-\frac{1}{x(x+1)(2 x+1)^{2}}<0 \end{array}

Therefore, g(x)g^{\prime}(x) is monotonically decreasing on (0,+)(0,+\infty).
Since limxg(x)=limxlnx+1xlimx22x+1=0\lim _{x \rightarrow \infty} g^{\prime}(x)=\lim _{x \rightarrow \infty} \ln \frac{x+1}{x}-\lim _{x \rightarrow \infty} \frac{2}{2 x+1}=0, and knowing that g(x)g^{\prime}(x) is positive on (0,+)(0,+\infty), it follows that g(x)g(x) is monotonically increasing on (0,+)(0,+\infty). Therefore, when nn is a positive integer, hn=eg(n)h_{n}=\mathrm{e}^{g(n)} is a strictly increasing sequence, i.e., h1<h2<<hn<hn+1h_{1}<h_{2}<\cdots<h_{n}<h_{n+1}.

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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.