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Inequality with integers and indices
Michael Niland   7
N Mar 18, 2025 by Ninjametry
Without using a calculator, show that $2^{\frac{1}{2}}$ $< 3^{\frac{1}{3}}$, and for a positive integer $n\geq3$, prove that

$n^{\frac{1}{n}}$ $>(n+1)^{\frac{1}{n+1}}$.
7 replies
Michael Niland
Mar 17, 2025
Ninjametry
Mar 18, 2025
Inequality with integers and indices
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Michael Niland
681 posts
#1
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Without using a calculator, show that $2^{\frac{1}{2}}$ $< 3^{\frac{1}{3}}$, and for a positive integer $n\geq3$, prove that

$n^{\frac{1}{n}}$ $>(n+1)^{\frac{1}{n+1}}$.
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invisibleman
13 posts
#2
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In order to solve the first problem, I have the following idea: raise both sides to the 6th power, and you're done! Right?
In the general case, consider the function f(x)=lnx/x, whose derivative is negative for x>e, so the function is decreasing. Let x=n and x=n+1, and compare them! I think you understand, if not, write and I'll answer!
This post has been edited 3 times. Last edited by invisibleman, Mar 18, 2025, 11:36 AM
Reason: typo
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krish6_9
18 posts
#3
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The inequality is equivalent to $n^{n+1} > (n+1)^n$. Taking natural logarithm on both sides, this is equivalent to $(n+1) \ln n > n \ln (n+1).$ Dividing, this is equivalent to $\frac{n+1}{\ln (n+1)} > \frac{n}{\ln n}$. Since $\frac{x}{\ln x}$ is increasing (obviously linear functions increase more quickly than logarithms, or using the derivative like @above said), we are done by definition.

i hope this is right! :D :ddr:
well someone taught me the solution hehe
This post has been edited 2 times. Last edited by krish6_9, Mar 18, 2025, 12:31 AM
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Speedysolver1
80 posts
#4
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Help me with a forum
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krish6_9
18 posts
#5
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who, me? wut does that mean
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ohiorizzler1434
717 posts
#6
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Bro! Let's show n^(n+1) > (n+1)^n , or (1+1/n)^n < n. Note that (1+1/n)^n < e because this limits upwards towards e. So for n >= 3, this is obviously true as e<3. Bruh! Now that's rizz!
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allisok100
1 post
#7
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f(x)=\frac{\ln x}{x}\Rightarrow f'(x)=\frac{1-\ln x}{{{x}^{2}}}<0,\forall x>e\Rightarrow f(n)>f(n+1),\forall n\ge 2
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Ninjametry
284 posts
#8
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Graph x^(1/x)
now observe where we get maxima
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