Is the following the only reason why |x| ≠ ±x?

  1. Assumption: |x| is unconditionally equal to ±x.

    This makes sense because if you take a look at a graph of y=|x|, and plot any horizontal line y=C where C is some constant, you will always have two solutions: one is positive and one is negative.

    But if we substitute any number into x, then we realize that this actually contradicts:

    |x| = ±x
    Let x = 2
    |2| = ±2
    2 = ±2
    2 = 2 OR 2 = -2

    Am I missing something or is the only reason why they aren't unconditionally equal?
    Last edited: Sep 3, 2013
  2. jcsd
  3. CompuChip

    CompuChip 4,299
    Science Advisor
    Homework Helper

    That final statement is true, isn't it? So I don't see an issue there.

    I think the problem you are running into is that "±x" isn't well-defined notation, whereas |x| is unambiguously defined. People often use it as shorthand, as you have done, for example in statements like "The solution of x² = 4 is x = ±2", but that is just an informal way of saying "The solutions of x² = 4 are x = -2 and x = +2".

    You could write "The solution of x² = 4 is |x| = 2" but that is technically something different - what you are saying then is: "The solutions to the equation x² = 4 are the same as the solutions to the equation |x| = 2" (and the solutions to both equations are x = 2 and x = -2).
  4. the modulus of x equals x if x is non-negative and -x if x is less than zero. It does not equal ±x.

    The equation mod(x)=2 has the solutions x= ± 2
  5. Opps. I've corrected the mistake.

    has been changed to:

    since ±2 is positive 2 AND negative 2.

    Hmm... interesting perspective. I suppose it might be a syntax issue.

    You mean absolute value function instead of modulus function right?

    The issue is that you don't know if x is negative or non-negative.
  6. CompuChip

    CompuChip 4,299
    Science Advisor
    Homework Helper

    No, it can only have one value. "x = 2 and x = -2" does not make sense, as a variable can only have one value at the time.
    As I said, it is usually used as shorthand for "+2 or -2".
  7. Hmm... you're right. I changed it back.
  8. CompuChip

    CompuChip 4,299
    Science Advisor
    Homework Helper

    That is the same with x in ##x^2=4##. Is it an issue for you there?
  9. Hmm...., right again.

    How does it not equal ±x? It's equal to +x or -x depending on whether x is non-negative or negative.
  10. D H

    Staff: Mentor

    Because ±x is a multivalued function of x with two branches while |x| is a true function of x. Note that the term "multivalued function" is a bit of a misnomer. A multivalued function is not a function.
  11. So the difference is that |x| has the condition and gives you the right solution depending on the condition and ±x just says either +x OR -x but it doesn't give you the condition?
  12. Mark44

    Staff: Mentor

    No, the difference is that |x| represents a single number. ±x represents two numbers, as long as x isn't 0.
  13. CompuChip

    CompuChip 4,299
    Science Advisor
    Homework Helper

    Can I give you some advice?

    Actually, I'm going to do it anyway :-p

    As long as you don't completely understand "±x", avoid using it. As I pointed out before, it does not have any formal definition like |x| does - it is merely used as shorthand. For the time being, I would suggest that you focus on getting the basics right. Writing "x = -2 V x = 2" is hardly more work than "x = ±2", it is unambiguous and it doesn't confuse anyone, including yourself.

    Once you have properly learned about functions and branch cuts you may be more sloppy :-)
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Draft saved Draft deleted