Tuesday, 5 November 2013

Proving an inequality involving the logarithm function: $frac{1}{n+1} leq ln left(1+frac 1nright) leq frac 1n$




The question is to prove the inequality $$\frac{1}{n+1} \leq \ln \left(1+\frac 1n\right) \leq \frac 1n\\\forall n \geq 1, n\in \mathbb N$$



I tried using Taylor expansion but couldn't figure out anything. Any ideas? Thanks.


Answer



Suppose $n$$ \frac{1}{n+1}<\frac{1}{x}<\frac{1}{n}. $$
Integrate this with respect to $x$, from $n$ to $n+1$. Then

$$ \frac{1}{n+1}<\log{(n+1)}-\log{n}<\frac{1}{n}, $$
and the middle is $\log{(1+\frac{1}{n})}$.


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