I want to show that $$\frac{x}{1+x}<\log(1+x)
x1+x<log(1+x)⇔x1+x−log(1+x)<0
Let f(x)=x1+x−log(1+x). Since f(0)=0 and f′(x)=1(1+x)2−11+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)x−a⇔1x0+1=log(x+1)x.
Since x0>0⇒1x0+1<1. ⇒1>1x0+1=log(x+1)x⇒x>log(x+1)
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