1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Seeking help for limit math problem

  1. Sep 19, 2007 #1
    lim (sqrt(x^2+5) - 3)/(x-2)
    x->2


    i hope that kinda makes sense...

    anyways, my question is that ive been doing this and i keep getting 0 in the denominator in the answer...after subsituting everything i got 4/ sqrt(9) - 3 = 4/0
    so then is this equation just not possible?
     
    Last edited: Sep 19, 2007
  2. jcsd
  3. Sep 19, 2007 #2

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    How did you get this? That doesn't appear to bear any relation to the initial expression you gave.
     
  4. Sep 19, 2007 #3
    For [tex]\lim_{x \rightarrow 2}\frac{\sqrt{x^2+5}-3}{x-2}[/tex]

    Clearly the substitution method will give you a zero denominator!

    So, what methods have you learned to try and "avoid" a zero denominator?

    Casey

    I think they meant (sqrt(4+5)-3)/0....
    Arithmetic errors are always a good start:wink:
     
    Last edited: Sep 19, 2007
  5. Sep 19, 2007 #4
    um the course im taking told me first multiply the numerator and denominator by
    sqrt(x^2+5) - 3

    i might have done it wrong but then i got x^2+5-9/x-2(sqrt(x^2+5) - 3)
    i factored that to x^2-4/x-2(sqrt(x^2+5) - 3)
    and then eventually got (x-2)(x+2)/x-2(sqrt(x^2+5) - 3)
    canceled out x-2
    x+2/sqrt(x^2 + 5) - 3
    then subtituted the x for 2
    thats how i got that answer....i think thats how the teacher told me to do it...probably made mistakes along the way though..im not sure
     
  6. Sep 19, 2007 #5

    EnumaElish

    User Avatar
    Science Advisor
    Homework Helper

    Where is the interaction term in your numerator? How do you expand (a + b)^2?
     
  7. Sep 19, 2007 #6
    Your on the right track by using the conjugate; however, you need to watch your signs
    the conjugate of [tex]\sqrt{x^2+5}-3[/tex] is [tex]\sqrt{x^2+5}+3[/tex]

    Casey

    PS~Do you understand why the sign needs to be a plus (+) sign?
    Hint: [tex](a+b)^2 [/tex] does not = [tex]a^2+b^2[/tex]
    Casey
     
    Last edited: Sep 19, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Seeking help for limit math problem
Loading...