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

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The discussion centers on the equivalence of the expressions 1/(1+e^-(x)) and 1-e^-(x) for very large values of x, specifically when x > 50. The original poster argues that their use of this approximation in a theorem about hole density in semiconductors was marked incorrect, despite the limiting behavior showing they are nearly equal as x approaches infinity. They emphasize that the approximation is valid and question the grading decision, suggesting that the marker may have expected a more detailed justification. The conversation highlights the importance of understanding approximations in mathematical proofs, particularly in the context of semiconductor theory. The poster advocates for clarification from the grader regarding the marking criteria.
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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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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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