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Limit problem

  1. Jan 31, 2015 #1
    1. The problem statement, all variables and given/known data
    If Lim f(x) and Lim g(x) both exist and are equal
    x→a x→a then Lim[f(x)g(x)]=1
    x→a
    2. Relevant equations
    No relevant equations are required in this problem. To determine whether the statement is true or false



    3. The attempt at a solution
    The statement is false but the reason behind it is quite unclear to me. Is it because when the limit approaches a from left and right the limits are -∞ and +∞?? Would be very helpful if anyone of you could explain it.
     
  2. jcsd
  3. Jan 31, 2015 #2

    BvU

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    Hard to read your post. Something like

    If ##\displaystyle \lim_{x\rightarrow a} f(x)## and ##\displaystyle \lim_{x\rightarrow a} g(x)## both exist and are equal ##\Rightarrow \displaystyle \lim_{x\rightarrow a} f(x)g(x) = 1 ## ?

    Could that be ##\displaystyle \lim_{x\rightarrow a} f(x)/g(x) = 1 ## ?

    • To me ##\displaystyle \lim_{x\rightarrow a}## means "x approaching from the left to a" so I wouldn't worry about "coming from the right"
    • To me ##\displaystyle \lim_{x\rightarrow a} f(x) = \pm \infty## means the limit does not exist, so I wouldn't worry about those either
    • But the division cause a problem in a particular case, that can serve as a counter-example that makes the general statement not true,
     
    Last edited: Jan 31, 2015
  4. Jan 31, 2015 #3

    Mark44

    Staff: Mentor

    The notation ##\lim_{x \to a} f(x)## indicates a two-sided limit in which x can approach a from either side. If the two-sided limit exists, then both one-sided limits also exist. Maybe that's what you were trying to say, but what you actually said wasn't clear.
    I agree that the first post was unclear, and that the OP meant division instead of multiplication.
     
  5. Jan 31, 2015 #4

    Ray Vickson

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    Standard usage is that ##x \to a## means a two-sided limit (##|x-a| \to 0##). Left-hand limits are typically denoted as ##x \to a\!-## or ##x \to a\!-\!0## or ##x \uparrow a##. Right-hand limits are typically denoted as ##x \to a\!+## or ##x \to a\!+\!0## or ##x \downarrow a##.
     
  6. Jan 31, 2015 #5

    BvU

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    Good thing you corrected me, gentlemen. Been too long ago, but I do recognize the ##x\downarrow a## and ##x\uparrow a\;.\ ## Never had much opportunity to make good use of the distinction. Apologies to professor D.
     
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