Saturday, 14 June 2014

calculus - Showing $frac{x}{1+x}

I want to show that $$\frac{x}{1+x}<\log(1+x)0$ using the mean value theorem. I tried to prove the two inequalities separately.



x1+x<log(1+x)x1+xlog(1+x)<0



Let f(x)=x1+xlog(1+x). Since f(0)=0 and f(x)=1(1+x)211+x<0 for all x>0, f(x)<0 for all x>0. Is this correct so far?



I go on with the second part:
Let f(x)=log(x+1). Choose a=0 and x>0 so that there is, according to the mean value theorem, an x0 between a and x with



f(x0)=f(x)f(a)xa1x0+1=log(x+1)x.




Since x0>01x0+1<1. 1>1x0+1=log(x+1)xx>log(x+1)

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