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The limit as x approaches infinity

  1. Jun 9, 2008 #1
    1. The problem statement, all variables and given/known data
    (3x-2) / (9x+7)

    As x approaches infinity.

    3. The attempt at a solution
    I know the procedure, but am then stuck:
    - Rearrange
    - Plug in infinity for x
    - Evaluate

    Tried breaking it into 3x / (9x+7) - 2 / (9x+7)
    The second thing would go to zero.
    The first, no clue.

    Maybe multiplying the original problem by something? Unsure what.

    (Note: This is to help a friend. I myself am going to bed now, but they will be checking for replies. They're not allowed to use L'Hopital's rule, as it wasn't taught)
     
    Last edited: Jun 9, 2008
  2. jcsd
  3. Jun 9, 2008 #2
    factor out x from both num and den!
    and plug in the infinity value without thinking
     
  4. Jun 10, 2008 #3

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    No, you don't "know the procedure". Only in the most trivial limits (limits of continuous functions where, by definition, the limit is the value of the function, can)you "plug" the target point into the formula. And certainly you can't "plug infinity" into this formula because it is only defined for real numbers and "infinity" is not a real number.

    It is true, however, that limit, as "x goes to infinity", of 1/x is 0 and 0 is easy to use. In order to change from "x" to "1/x" divide both numerator and denominator by x (that is the same as rootX's "factor x from numerator and denominator").
     
    Last edited: Jun 10, 2008
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