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Question about modular functions

  1. Aug 20, 2010 #1
    Suppose I have any modular function, for example:

    [tex]f(x) = |2x + 4| + 3[/tex]

    I can rewrite the function in the following way:

    [tex]f(x) = \left\{\begin{matrix}
    2x + 7, \;\; x \geq -2\\
    -2x -1, \;\; x < -2



    Okay, the question is: how do I know that the first condition is [tex]\geq[/tex] and second condition is < and not vice-versa?

    Thank you,
    Rafael Andreatta
  2. jcsd
  3. Aug 20, 2010 #2
    It doesn't matter, because |x| is a continuous function. What's the value of f(x) for x=-2?
  4. Aug 20, 2010 #3
    1. You mean, because |x| is a continuous function or because f is a continuous function?

    2. What if it was not continuous?
  5. Aug 20, 2010 #4


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    f(x) is a continuous function, so the two halves of the definition must agree where they meet up.

    Since adding and multiplying and composing continuous functions is a continuous function, and sum of absolute value functions like f(x) here will be continuous. In the event you're doing something like dividing by the absolute value of x, then you can just:
    plug in the value of x for which you're unsure and compare it to your formulae. See which one it agrees with. If the function isn't defined for that value of x, then you don't need to decide which inequality gets the equal sign because you aren't defining the function anyway
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