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Algebra Difficulty 4.0 AIME Find the answer

Given the inequality x+mlnx+1exxmx+m\ln x+\frac{1}{e^{x}}\geq x^{m} holds for x(1,+)x\in (1,+\infty), then the minimum value of the real number mm is ____.

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve the given problem, we start by analyzing the given inequality x+mlnx+1exxmx+m\ln x+\frac{1}{e^{x}}\geq x^{m} for x(1,+)x\in (1,+\infty). We can rearrange this inequality to better understand its components:

x+1exxmmlnx=xmlnxm x+\frac{1}{e^{x}}\geq x^{m}-m\ln x=x^{m}-\ln x^{m}

This rearrangement allows us to compare the left side of the inequality, which is a sum of xx and a decreasing function 1ex\frac{1}{e^{x}}, with the right side, which is a transformation of xmx^{m}.

Next, we introduce a function f(x)=xlnxf(x)=x-\ln x and find its derivative to understand its behavior:

f(x)=11x=x1x f'(x)=1-\frac{1}{x}=\frac{x-1}{x}

The derivative shows that f(x)f(x) is increasing for x>1x>1 and decreasing for x1x1, f(ex)f(xm)f(e^{-x})\geq f(x^{m}) because 0101. When m=0m=0, xm=1x^{m}=1, and it's clear that f(ex)f(xm)f(e^{-x})\geq f(x^{m}) holds.

To find the minimum value of mm, we consider the case when m1m1, which implies exxme^{-x}\leq x^{m}. Taking the natural logarithm of both sides gives us xmlnx-x\leq m\ln x, leading to mxlnx-m\leq \frac{x}{\ln x} for x>1x>1.

We then analyze the function h(x)=xlnxh(x)=\frac{x}{\ln x} to find its behavior:

h(x)=lnx1(lnx)2 h'(x)=\frac{\ln x-1}{(\ln x)^{2}}

This derivative shows that h(x)h(x) is increasing for x>ex>e and decreasing for 1<x<e1<x<e, with a local minimum at x=ex=e. The value of h(x)h(x) at this minimum is ee, which implies me-m\leq e.

Therefore, we conclude that the minimum value of mm is e-e, encapsulating this final answer as:

e \boxed{-e}

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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.