Wednesday, 23 November 2016

real analysis - Show $1-1/x


Show $1-1/x <\ln(x) for all $x>1$?





My attempt:



(1) $\ln (x)



Suppose $h: \mathbb{R}^+ \to \mathbb{R}, t\mapsto t-1-\ln(t)$, then $h'(t)=0$ if $t=1$. Furthermore $h'(t)>0$ for all $t\in \mathbb{R}^+$.



(2) $1-1/x<\ln(x)$



Suppose $f: \mathbb{R}^+\to \mathbb{R}, t\mapsto \ln(t)-1+\frac{1}{t}$, then $f'(t)=0$ for $t=1$ and $f'(t)>0$ for all $t\in \mathbb{R}^+$.




Therefore the inequality is true.

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