Tuesday, 5 November 2013

Proving an inequality involving the logarithm function: frac1n+1leqlnleft(1+frac1nright)leqfrac1n




The question is to prove the inequality 1n+1ln(1+1n)1nn1,nN



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


Answer



Suppose $n1n+1<1x<1n.


Integrate this with respect to x, from n to n+1. Then

1n+1<log(n+1)logn<1n,

and the middle is log(1+1n).


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