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Limit Problem

  1. Sep 30, 2012 #1
    1. The problem statement, all variables and given/known data

    Find algebraically:

    [tex]\lim_{x\rightarrow 7} {\frac{3x}{\sqrt{x+2}}} [/tex]

    2. Relevant equations

    None

    3. The attempt at a solution

    [tex] {\frac{3x}{\sqrt{x+2}}} * \frac{\sqrt{x+2}}{\sqrt{x+2}}[/tex]
    [tex] \frac{3x\sqrt{x+2}}{x+2}[/tex]

    In most of these problems, I get the limit of a square root + another number and I'm able to factor. This problem just threw me off. How should I proceed?
     
    Last edited: Sep 30, 2012
  2. jcsd
  3. Sep 30, 2012 #2
    When you are asked to find limits such as this, it is usually the case that the expression is not continuous at the value of x that you are asked to find the limit at.

    Can you see where this function is not continuous?
     
  4. Sep 30, 2012 #3
    No graphing calculators. This has to be done algebraically. I'll add that to the thread.
     
  5. Sep 30, 2012 #4
    No graphing calculators are required. Do you know what continuous means?

    What makes you think you need to do what you did in your "attempt at a solution"?
     
  6. Sep 30, 2012 #5

    In order to see what part of the function is continuous, you need to be able to see a graph. I would have to graph it or visualize it in my head. Unfortunately, I cannot visualize the graph and therefore cannot tell where it's continuous or not.

    I have to do it "algebraically" according to my textbook and teacher. She doesn't want us to just look at the function and point out the limit.
     
  7. Sep 30, 2012 #6
    Ok, what year are you in? Sorry, I have to ask this to judge my comments for a homework question.

    I ask this as if you are being asked to find limits, I would've thought you'd know what continuous meant.

    Can you see what this function equals if x=7?
     
  8. Sep 30, 2012 #7
    No no no I know what continuous means. This is for my Calc class senior year. We're doing pre calc review and I'm a little rusty on a few things.

    If x=7 it's undefined. From my understanding, when you are solving in the manner my teacher has requested, you usually either factor, rationalize, or something along those lines until you have an equation where you can plug in the value x is approaching and find a limit.
     
  9. Sep 30, 2012 #8
    What makes you say that?
     
  10. Sep 30, 2012 #9
    Oh wait this whole thread has been incredibly stupid. I just realized that I completely read the problem and everything you've been saying wrong and that this problem didn't call for me to change the equation given the value x is approaching can be plugged in. I just automatically assumed it couldn't given that all the problems I had before were much harder and required that. Lmao this what happens when your brain is put into too much of a routine; you don't even read the problem. That and the lack of sleep I've had this weekend.

    I'm requesting a lock on this thread. Issue has been resolved.
     
  11. Sep 30, 2012 #10
    No problem! :smile:

    Everyone has days like that! :smile:

    Don't be afraid to come back with your next problem, though!
     
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