Friday, 24 October 2014

real analysis - Using the Law of the Mean (MVT) to prove the inequality $log(1+x)

If $x \gt0$, then $\log(1+x) \lt x$.



My attempt at the proof thus far...



Let $f(x) = x-\log(1+x)$, then $f'(x) = 1-\frac{1}{1+x}$ for some $\mu \in (0,x)$ (since $x>0$)



MVT give us $f(x) = f(a) + f'(\mu)(x-a)$




So plug in our values we get:



$$x-\log(1+x) = 0+(1-\frac{1}{1+\mu})(x-0)$$



which we can reduce to



$$\log(1+x)=\frac{x}{1+\mu}$$



Now For $x\leq1$, then $0\lt\mu\lt1$




S0 $0\lt \frac{1}{1+\mu}\lt 1$, thus $\log(1+x)\lt x$



If $x>1$, then....



So I can see clearly that if $x>1$ is plugged into $\frac{x}{1+\mu}$



then $\log(1+x)

I would appreciate tips hints or proof completions.

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