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: Integration using exponentials 2

  1. Oct 17, 2012 #1
    1. The problem statement, all variables and given/known data


    For this forum they use the following integration:


    3. The attempt at a solution

    Where does (u+a) come from? In step 2 why does the side on the left of the plus sign become zero? In step 3, how does A(0+a√π/λ) = a

    This website likes you to work on problems but I can't work on something for which step 1 is a mystery.
  2. jcsd
  3. Oct 17, 2012 #2
    They do a change of variables, x-a = u, so x= u+a

    The easiest way to see this is to notice that the integrand is an odd function (that is, f(-x) = -f(x)) and therefore the integral over an even interval [-n,n] is zero.

    Did they calculate the value for A before? What you are doing here is calculating an expectation value of a random variable, so it should be true that
    [tex] \int_{-\infty}^\infty dx Ae^{-\lambda(x-a)^2} dx = 1 [/tex]

    Yes but you can't explain every single operation you do. It just seems to me you're trying to do assignments which are a bit too difficult for you. Maybe start from something easier?
  4. Oct 17, 2012 #3
    Thanks for your answers.

    Let's say my motivation to learn math and science right now (I'm into the humanities) on a scale of 1 to 10 is about 3. I don't have the motivation to go back and learn more calculus. I just want to see how much QM I can get through with the calc I have. Then after about 2 years hopefully I'll be able to make another effort to learn math and physics with a huge burst of motivation. This time around I dedicated about 1000 hours towards math and physics, after a two year break I'll put forth another 1000 hour effort if I have the time.
  5. Oct 17, 2012 #4
    But it does not speed things up if you jump over things you haven't learned/have forgotten/whatever. In fact it's likely it's making your learning slower.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook