The sum of these functions equals a constant

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    Constant Functions Sum
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Discussion Overview

The discussion revolves around the implications of the equation ##f(x) + g(x) = c##, where ##c## is a constant. Participants explore whether this relationship necessitates that both functions ##f(x)## and ##g(x)## must also be constants, and they examine specific examples to illustrate their points.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that if ##f(x) + g(x) = c## holds for all ##x##, then both functions must be constants.
  • Another participant counters this by providing an example with non-constant functions, ##f(x) = \cos x## and ##g(x) = -\cos x + 1##, which sum to a constant but are not constants themselves.
  • The second participant emphasizes that the nature of the equation (whether it is an identity or holds for specific values of ##x##) affects the interpretation of the functions involved.
  • One participant expresses confusion about the implications of having two arbitrary functions whose sum is zero, initially thinking it implies both functions must be zero.
  • Another participant clarifies that if ##f(x) + g(x) = 0##, then it follows that ##g(x) = -f(x)##, but does not imply that both functions are zero.

Areas of Agreement / Disagreement

Participants do not reach a consensus. There are competing views on whether the sum of two functions equating to a constant necessitates that the functions themselves are constants.

Contextual Notes

Participants note that the interpretation of the equation depends on whether it is considered an identity or valid for specific values of ##x##. There is also a mention of the implications of arbitrary functions and their sums.

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If I have a sum ##f(x) + g(x) = c##, with ##c## a constant, does this imply that both ##f(x)## and ##g(x)## are also constants?
If I just solve this equation for ##x##, I will find some values of ##x## which satisfy the equation. However, if I require that the equation be true for all ##x##, there is no way other than the functions being equal to constants. Am I right?
 
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kent davidge said:
If I have a sum ##f(x) + g(x) = c##, with ##c## a constant, does this imply that both ##f(x)## and ##g(x)## are also constants?
No, not at all. Consider ##f(x) = \cos x## and ##g(x) = -\cos x + 1##. Neither function is constant, but their sum is always equal to 1.
kent davidge said:
If I just solve this equation for ##x##, I will find some values of ##x## which satisfy the equation. However, if I require that the equation be true for all ##x##, there is no way other than the functions being equal to constants. Am I right?
No. With my example functions, you can't solve the equation ##f(x) + g(x) = 1## for x. My equation is true for all real x.

Also, it depends on whether the equation you're working with is true only for a certain number of values of x, or is an identity, one that is true for all values in the domains of the two functions.

Can you be more specific about what you're trying to do?
 
Mark44 said:
Can you be more specific about what you're trying to do?
I just have two seemgly arbritary functions of ##x## whose sum is zero, and wanted to know if that implies that the functions themselves are equal to zero, but now with your answer I see that it doesn't.
 
kent davidge said:
If I just solve this equation for x, I will find some values of x which satisfy the equation.
All x should satisfy the equation if that equation is a general relation ("c is a constant" implies that).
 
kent davidge said:
I just have two seemgly arbritary functions of ##x## whose sum is zero, and wanted to know if that implies that the functions themselves are equal to zero, but now with your answer I see that it doesn't.
If ##f(x) + g(x) = 0## then ##g(x) = -f(x)##.
 
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