Tuesday 25 November 2014

inequality - Prove $ln(n) lt n$ using only $log(x^y) = ylog(x) $




Prove $$\ln(n) \lt n \;\;(n\in \mathbb N) $$
Using only the rule $$\log(x^y) = y\log(x) $$
I tried using any of the known Inequalities am-gm, power mean, titu's lemma, holder's,... but I seem I can't go anywhere near required.
Any hint?



$\mathbf {Edit} $
It was simpler than I thought, it's done with Bernoulli's Inequality.
$$e^n\gt 2^n = (1+1)^n \ge 1+n \gt n$$
$$\ln(e^n) \gt \ln(n)$$
$$\ln(n) \lt n$$


Answer




Your inequality could be generalized as $$\ln x \leq x-1, x>0$$ To prove it, you perhaps have to use calculus, not just elementary mathematics.Notice that, for $f(x)=\ln x-x+1,(x>0)$, $$f'(x)=\frac{1}{x}-1.$$ Thus, $f'(x)>0,$ when $01.$ For the reason, $f(x)$ reaches its maximum value $f(1)=0$ at $x=0$, namely, $f(x) \leq 0$.



But if you have known Bernoulli's inequality, there indeed exists an elementary proof.



Since




$$(1+x)^n \geq 1+nx,(x \geq -1, n\in \mathbb{N_+}).$$





Let $x=e-1$. Then $$e^n \geq 1+n(e-1)=1+ne-n>n.$$



It follows that $$n>\ln n.$$


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