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!

Simple integration question involving-infty subscript

  1. Sep 17, 2011 #1
    simple integration question involving-infty "subscript"

    1. The problem statement, all variables and given/known data

    Been reading about signals, but my calculus skills have rusted (or never has been all that good in the first place).

    So ...

    2. Relevant equations

    Why does [itex]x(t) = \int^t_{-\infty} x'(\tau) \,d\tau[/itex] ?

    3. The attempt at a solution

    You will end up with [itex]x(\tau)|^t_{-\infty} = x(t) - x({-\infty})[/itex]. Right?

    So [itex]x({-\infty}) = 0[/itex] for all functions?

    Been looking on the web, but I have no idea how to google this.
     
  2. jcsd
  3. Sep 17, 2011 #2

    Mark44

    Staff: Mentor

    Re: simple integration question involving-infty "subscript"

    No, you can't do this. Infinity is not a number that you can substitute into a function. Your integral is one type of improper integral. To evaluate an integral like this, you need to work with a limit, like so:
    [itex]x(t) = \int^t_{-\infty} x'(\tau) \,d\tau = \lim_{a \to -\infty} \int_a^t x'(\tau) \,d\tau [/itex]
     
  4. Sep 18, 2011 #3
    Re: simple integration question involving-infty "subscript"

    Hmm ...

    Looks like my calculus really does suck.

    It was just written "like that" in the book. It probably assumes that I know how to solve it. ><

    Thanks.
     
    Last edited: Sep 18, 2011
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Simple integration question involving-infty subscript
Loading...