Monday, 8 October 2018

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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