Saturday, September 19, 2015

calculus - Prove with the mean value theorem that $x-frac{x^2}{2} < ln(1+x)$




Prove with the mean value theorem that $x-\frac{x^2}{2}<\ln(1+x)$(0,\infty)$



Approach
$f(x) := \ln(1+x) $ with the mean value theorem in $[0,x]$



$\frac{1}{1+\xi}= \frac{\ln(1+x)-0}{x-0}$




$\frac{1}{1+\xi}$ takes the biggest value when $\xi$ is $0$



and so $1 <\frac{\ln(1+x)}{x}$ multiply with x and you get
$x<\ln(1+x)$



I can't prove the second part.


Answer



For MVT



$$\ln (1+x)+\frac{x^2}{2}-(\ln 1+0)=x\cdot\left(\frac{1}{1+c}+c\right)>x \quad c\in(0,x)$$




Indeed



$$\frac{1}{1+c}+c=\frac{c^2+c+1}{1+c}>1$$


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