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How can there be a function with a second derivative greater than zero

  1. Sep 17, 2008 #1
    1. The problem statement, all variables and given/known data

    yet is approaching negative infiniti?

    We were discussing a quesiton that went "suppose f double prime (x)>0 and x ranges from negative infiniti to infiniti, and f(a)=0. Prove or disprove that f(x) is bounded below."

    The man said that e^-x - x had a second derivative greater than zero, which i understand, but then how does it go to negative infiniti? I don't understand that, it should go to positive infiniti, right?

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 17, 2008 #2

    Dick

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    If f(x)=exp(-x)-x, f(x) does go to positive infinity as x->-infinity. What happens if x->+infinity?
     
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