Sunday, October 9, 2016

real analysis - prove $log(1+x)0$

Prove $\log(1+x) \lt x$ for $x\gt0$


my attempt:


I show $e^{x}\gt 1+x$ for $x\gt0$ since


$e^{x}=1+x+\frac{n(n-1)}{2}\frac{x^2}{n^2}+...$


so if $x\gt0$ then all terms are positive


so $e^{x}\gt 1+x$ for $x\gt0$


now given $e^{x}\gt 1+x$ for $x\gt0$, can I take $\log$ on both sides and show


$\log(1+x)\lt x$ for $x\gt0$


or do I have to prove firstly that $\exp(x)=e^x$ for $x\gt 0$ then I can take $\log$..

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