1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    Science Advisor
    Homework Helper

    If f(x)=exp(-x)-x, f(x) does go to positive infinity as x->-infinity. What happens if x->+infinity?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook