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Lim x->0- |x| = -x

  1. Feb 24, 2005 #1
    This pretty book here says that [tex] \lim_{x\rightarrow 0^-}\frac {|x|}{x} [/tex] is equal to [tex] \lim_{x\rightarrow 0^-}\frac {-x}{x} [/tex] ...

    I understand that we're taking a left sided limit, as x approaches 0, so x will always be negative, but we're putting that value in the absolute value function which should always return a postive result. So what's the conceptual reason for the absolute value function returning a negative number? The only thing I could come up with is the order of operation application is different; you take the absolute value of x first and then let that x approach 0 from the left, which would be result in a negative x.
     
    Last edited: Feb 24, 2005
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  3. Feb 24, 2005 #2

    dextercioby

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    It doesn't.It returns a positive value ALWAYS...Even for complex #...

    Daniel.
     
  4. Feb 24, 2005 #3
    uhhhhh... then can you prove that [tex] \lim_{x\rightarrow 0}\frac {|x|}{x} [/tex] exists? :) My text, second edition Calculus concepts an contexts by james stewart, says it doesn't exist.
     
    Last edited: Feb 24, 2005
  5. Feb 24, 2005 #4

    dextercioby

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    You mean the left or the right limit...?The partial limits are - and + infinity respectively...So the limit -------->0 (full) DOES NOT EXIST...Neither in Rbar.

    Daniel.
     
  6. Feb 24, 2005 #5
    wiat a second, x can be negative; its the y value, the range, that is always positive. But even still it appears that they're abusing the || function because when you slap a function on a number you use the result, the range, not the input (otherwise there's no point in using the functino in the first place). For any of you that have this text, its on page 115.
     
  7. Feb 24, 2005 #6

    dextercioby

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    No,the limit is generally COMPUTED OVER THE DOMAIN OF DEFINITION.

    Daniel.
     
  8. Feb 24, 2005 #7
    says here the partial limits are +/- 1.

    If it said [tex] \lim_{x\rightarrow 0^-}\frac {|x|}{x} = \lim_{x\rightarrow 0^-}\frac {x}{-x} = -1 [/tex] i'd by that, but the text says [tex] \lim_{x\rightarrow 0^-}\frac {|x|}{x} = \lim_{x\rightarrow 0^-}\frac {-x}{x} [/tex]
     
  9. Feb 24, 2005 #8

    Hurkyl

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    You're asking why |x| = -x when x is negative? Isn't that part of its definition?
     
  10. Feb 24, 2005 #9
    yeah that makes sense, but if x is negative then why isn't the denominator negative as well, since it's x? x-->0- holds for both variables in the function |x|/x
     
  11. Feb 24, 2005 #10

    Hurkyl

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    x is negative when x < 0.
     
  12. Feb 24, 2005 #11

    dextercioby

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    That's because YOU EDITED YOUR POSTS.You had a square in the denominator... :wink: :rolleyes:

    Daniel.
     
  13. Feb 24, 2005 #12

    NateTG

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    Let's say that
    [tex]x=-5[/tex]
    or something like that.
    Then we have
    [tex]\frac{|x|}{x}=\frac{|(-5)|}{(-5)}=\frac{5}{-5}=\frac{-(-5)}{(-5)}=\frac{-x}{x}[/tex]

    Since [itex]x[/itex] is negative, the negation in the numerator makes the numerator positive, and the denominator is negative.
     
  14. Feb 24, 2005 #13
    dextercoiby - yeah that was a typo i removed; changes everything sorry about that.

    hurkyl sure -x is negative but how does that force |x|/x to be negative. If x is negative, i'd expect something like -x/-x beause 2) |x| is -x and for x we're taking x -> 0- for denominator x.
     
  15. Feb 24, 2005 #14
    okay. The way i'm looking at this process now is:

    |x| gives us -x
    ---
    x gives us plain old x

    then we actually take the limit
    and we get
    -(-x)
    ----
    - x

    which gives us

    -x/x = -1

    is that's what going on?
    [edited for conceptual misunderstandings]
     
    Last edited: Feb 24, 2005
  16. Feb 24, 2005 #15
    Just because x is negative doesn't mean you should put a negative sign in front of it.
     
  17. Feb 24, 2005 #16

    NateTG

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    You seem to be confused about the following fact:

    If x is negative, -x is not negative.

    Take a moment and look at the sentence I just wrote, and make sure you understand it. You appear to be confusing negation (an operation) with something being negative (a property).
     
  18. Feb 24, 2005 #17
    okay i got it now. thanks for all the help Peoples of Physics and Chemistry. (POPAC)
     
  19. Feb 24, 2005 #18
    hmm so -x is negation of x, not negative x, which is the negation of positive x?
    eidt - accidentally typed negative when ishouldve used negation
     
  20. Feb 24, 2005 #19

    NateTG

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    One common misconception is essentially that
    -x
    is thought of as what would be written as:
    -|x|

    If that makes any sense at all.
     
  21. Feb 24, 2005 #20

    HallsofIvy

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    If x is negative then, yes, |x|= -x when is positive because x itself is a negative number. [itex]\frac{|x|}{x}= \frac{-x}{x}[/itex] which is a positive number over a negative number. That's negative.
     
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