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Exponential function

  1. Dec 23, 2012 #1
    1. The problem statement, all variables and given/known data

    k is a real number and [tex]f_{k}(t)=e^{t}-1-t-k\frac{x^{2}}{2}[/tex]

    1- Show that : [tex](\forall x\epsilon\mathbb{R}):0\leq e^{x}-1-x[/tex]

    2- Show that : [tex](\forall x>0)(\exists k\epsilon\mathbb{R}^{+})(\exists d\epsilon[0,x]):f(x)=f''(d)=0[/tex]

    3-Conclude that [tex](\forall x\epsilon\mathbb{R}):|e^{x}-1-x|\leq\frac{x^{2}}{2}e^{|x|}[/tex]


    3. The attempt at a solution

    For number 1 i said f(x) =e^x-1-x and f'(x)=e^x-1 so if x >0 than f in increasing and if x<0 f is a decreasing function so f(x)>f(0) in both cases so therefore: that 0<e^x-1-x.
    Number 2 I don't know what to do can someone help please??
     
    Last edited: Dec 23, 2012
  2. jcsd
  3. Dec 23, 2012 #2

    mfb

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    2016 Award

    Staff: Mentor

    fk(x)=0 for a specific x is sufficient to fix k, so you can show the existence of such a k first (using (1) and x^2/2 > 0).
    You can calculate f'' and show the existence of such a d in a similar way afterwards.
     
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