Is 1/(1+e^-(x)) equivalent to 1-e^-(x) when x is very large?

In summary, the conversation involved discussing an exam question where the asker used an approximation to prove a theorem regarding hole density in semiconductors. They noticed the limiting behavior of the approximation as x goes from 50 to infinity and questioned why their answer was marked incorrect. The expert suggests asking the person who marked it incorrect for further clarification.
  • #1
Cilly28
1
0
Long story short, I said on an exam that...

1/(1+e^-(x)) =~(is nearly equal to) 1-e^-(x); note* x > 50 , they are also inversely proportional when x<-50 but that isn't specific to the problem

I used this to prove a theorem regarding hole density in semiconductors ""when x is very very large""...It was marked incorrectly but the question stated that the magnitude of x would be >50. I noticed the limiting behavior of this as it goes from 50 to infinity and they appear to be equal, especially when approximations are made in the 'books proof' to simplify the equation, mainly throwing low integers away that do not effect a number of much higher magnitude.

If I made a correct approximation and still obtained the same result equation, just because I did it differently shouldn't make it incorrect, would anyone agree?
 
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  • #2
The apporximation is correct. I suggest you ask the person who marked it incorrect explain why. It may be that you were supposed to justify the approximation in more detail.
 

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