drwls & Damon - maths

hey guys,

i follow you up to the point of intergrating the final line. How would I intergrate [x^-2.e^-x] dx


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  1. My first suggestion was to try the method of "integration by parts", but that may not work. I found a recursion formula for the integral in my Table of Integrals, but it ends up requiring the integral of e^-x/x , which is a new fucntion called the exponential integral, Ei(x). Perhaps I made a mistake somewhere applying the "integrating factor" method. Damon has made more progress and is better equipped to help you. I am giving up on it

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    posted by drwls
  2. Yes, use the recursion formula. You need it later to cancel the next term.

    in general:
    int e^ax dx/x^n = (1/(n-1)) [-e^ax/x^(n-1) + a int e^ax/x^(n-1)dx] for n integergreater than one
    here n is two
    so here I had:
    y e^-x/x=int( e^-x dx/x)(1/x + 1)dx + C

    ye^-x/x =int e^-x dx/x^2 + int e^-x dx/x

    ye^-x / x = (1/1)[-e^-x/x - int e^-x dx/x ] + e^-x dx/x +C
    see that gets rid of the int e^-x dx/x terms and you are left with
    y e^-x/x = -e^-x/x + C
    y = -1 + x C / e^-x
    y = x C e^x - 1 :)

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    posted by Damon
  3. Typo:

    ye^-x / x = (1/1)[-e^-x/x - int e^-x dx/x ] + INTEGRAL e^-x dx/x +C
    see that gets rid of the int e^-x dx/x terms and you are left with

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    posted by Damon

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