1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Any integrating genius? integrate this

  1. Oct 18, 2013 #1
    this may seem simple, but try doing this yourself. i've tried sustituting t=e^x , e^-x. but the problem lies after that. do it and see it for yourself.

    Attached Files:

    • 121.PNG
      File size:
      7.1 KB
  2. jcsd
  3. Oct 18, 2013 #2

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    You aren't going to be able to solve this integral in terms of the elementary functions.

    Try the substitution t=e-z2.
  4. Oct 18, 2013 #3
    i think u are wrong, this doesn't solve the problem. the problem really lies in applying the limits in the end, not in the substitution.
  5. Oct 18, 2013 #4
    I'd like to elaborate on D H's suggestion a little bit.

    You should note that there is an explicit mention of [tex]e[/tex]. Also the possible answers include the square root of pi. Shouldn't that ring a bell?

    At least, this depends on your level of education. Try searching for gaussian integral. This might clear some things up for you.

    It does solve the integral check it. It took me about the size of a postcard and 1 minute to find the answer.
    Last edited: Oct 18, 2013
  6. Oct 18, 2013 #5
    dear jorisL,
    i'm not aware of gaussian integral. i'll definitely check it out. but, pls tell me, how did you come to conclusion that it can be solved by gaussian integral.
    when DH suggested e^-z^2 . i thoght it just meant i needed to sub t= e^-z^2.
  7. Oct 18, 2013 #6

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Sure it does. There is a very simple relationship between the integral you asked about in the original post and the following integral:
    [tex]\operatorname{erf}(x) \equiv \frac 2 {\sqrt{\pi}} \int_0^x e^{-t^2} dt[/tex]
    In particular, there's a direct relationship between your integral and erf(∞).

    Try as hard as you can and you will not be able to express either ##\int e^{-t^2}dt## or ##\int \frac{dt}{\sqrt{-\ln t}}## in terms of the elementary functions. Make all the u-substitutions you can think of. It won't work. Neither ##e^{-t^2}## nor ##\frac 1 {\sqrt{-\ln t}}## are integrable in terms of the elementary functions.

    That does not mean these functions don't have an integral. It just means you can't express those integrals in terms of the elementary functions.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted