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

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SUMMARY

The discussion centers on the mathematical function f(x) = e-x - x, which has a second derivative f''(x) > 0, indicating that it is concave up. Despite this, the function approaches negative infinity as x approaches positive infinity. The key conclusion is that a function can have a positive second derivative while still being unbounded below, as demonstrated by the behavior of f(x) in the specified limits.

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Homework Statement



yet is approaching negative infiniti?

We were discussing a question 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?

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The Attempt at a Solution

 
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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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