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Evaluate the limit as y approaches 4

  1. Mar 15, 2012 #1
    1. The problem statement, all variables and given/known data

    Evaluate lim y→4

    y-3√y + 2
    √y - 2

    3. The attempt at a solution

    If I plug 4 into Y, my answer is undefined. But if I do the chart method, where I plug in 3.999 and 4.0001 the answer is 1. So I'm not quite sure what I'm doing wrong.
  2. jcsd
  3. Mar 15, 2012 #2
    Try factoring the numerator! The square roots might throw you off a bit, but it's in the same form of a quadratic equation.

    You can't plug in 4 directly because the function isn't defined at 4. But limits don't concern themselves with what happens at that point, only what happens very close to that point.
  4. Mar 15, 2012 #3


    Staff: Mentor

    The numerator is quadratic in form, and can be factored.

    Or, you can let u = √y, and rewrite the fraction in terms of u, and factor the numerator. With this change, the limit will be as u → 2.
  5. Mar 15, 2012 #4
    Mark and Scurty, thank you. Mark I get your second method, but for both of you, I don't understand how to factor the numerator. Is it really factor-able? I dont get what value to factor by.
    Last edited: Mar 15, 2012
  6. Mar 15, 2012 #5


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    Staff Emeritus
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    Of course 4.001 & 3.999 won't give you exactly 1. They give 0.99975 and 1.00025 respectively.
  7. Mar 15, 2012 #6
    Can you use parenthesis? I can't tell what your sqrt is encompassing.
  8. Mar 15, 2012 #7
    Just the Y. Nothing else.
  9. Mar 15, 2012 #8


    Staff: Mentor

    Parentheses aren't needed. √y is just the square root of y.
  10. Mar 15, 2012 #9
    Can someone tell me how exactly do I factor the numerator?
  11. Mar 15, 2012 #10


    Staff: Mentor

    (√y - ?)(√y - ?)

    Like that...
  12. Mar 15, 2012 #11


    Staff: Mentor

    Or if you use my suggestion, you're factoring u2 - 3u + 2.
  13. Mar 15, 2012 #12
    I totally forgot factoring square roots. Thanks. Saved me alotta trouble.
    Last edited: Mar 15, 2012
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