Question #190541

Use the sign of the derivative to establish the inequaltiy ln(1 + x) > x − x 2 2 , ∀ x > 0


1
Expert's answer
2021-05-10T14:22:04-0400

Define function f(x)f(x) as:

f(x)=x22+ln(1+x)xf(x)=\frac{x^2}{2}+ln(1+x)-x


Note that: f(0)=0f(0)=0


Let us find first derivative of function f(x)f(x) :

f(x)=x2x+1 ,f'(x)=\frac{x^2}{x+1}\ ,


Note that f(x)>0f'(x)>0 for all x>0x>0


Since f(0)=0f(0)=0 and f(x)>0f'(x)>0 for all x>0x>0 it follows that:


x22+ln(1+x)x>0ln(1+x)>xx22\frac{x^2}{2}+ln(1+x)-x>0\Rightarrow ln(1+x)>x-\frac{x^2}2{} , for all x>0x>0


Hence Proved.


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