Wednesday, 30 April 2014

inequality - How can I prove that $x-{x^2over2}



How can I prove that $$\displaystyle x-\frac {x^2} 2 < \ln(1+x)$$ for any $x>0$




I think it's somehow related to Taylor expansion of natural logarithm, when:



$$\displaystyle \ln(1+x)=\color{red}{x-\frac {x^2}2}+\frac {x^3}3 -\cdots$$



Can you please show me how? Thanks.


Answer



Hint:



Prove that $\ln(1 + x) - x + \dfrac{x^2}2$ is strictly increasing for $x > 0$.




edit: to see why this isn't a complete proof, consider $x^2 - 1$ for $x > 0$. It's strictly increasing; does that show that $x^2 > 1$? I hope not, because it's not true!


No comments:

Post a Comment

real analysis - How to find $lim_{hrightarrow 0}frac{sin(ha)}{h}$

How to find $\lim_{h\rightarrow 0}\frac{\sin(ha)}{h}$ without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...