Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Limitsproving they exist?

  1. Apr 25, 2004 #1
    limits..proving they exist???

    Wot do u have to do to prove that an intergral exists.?? I know how to do it if the integrals bounds are given ( example, [a,b]) but wot if the integral is from x till infinity??
  2. jcsd
  3. Apr 25, 2004 #2

    matt grime

    User Avatar
    Science Advisor
    Homework Helper

    In the same wasy as infinite sums, work out the integral from a to b and then let b tend to infinity. Eg
    integral of 1/x from a to b is log(b) - log(a), which tends to infinity as b tends to infinity so the integral doesn't exist.
    integral of 1/x^2 from a to be is 1/a^2-1/b^2, which tends to 1/a^2 as b tends to infinity so the infinite integral exists.

    If you wish to integrate from minus infinity to infinity, you must do the integral from a to b and let a and b tend to infinity independently.

    Thus the improper integral of sin(x) over the real line does not exist even though you can choose the interval to be [-a,a] and get an answer of zero (other choices will give different answers hence the integral does not exist)
  4. Apr 25, 2004 #3
    How will you do
    from zero to infinity.
    Which can be written as a alternating series
    T subscript n =[tex]\mid\int\frac{sinx}{x}dx\mid[/tex] over intervals ([tex](n-1)\pi,n\pi[/tex])
    but how do show as n tends to infinity that T(n) tends to 0???
    cos i cant integrate it
    Last edited: Apr 25, 2004
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Limitsproving they exist?