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

I Miscellaneous Integration Technique

  1. Mar 20, 2016 #1
    We were taught of several integration technique, only to find one of those techniques came up as years of solving of our professor.. Can someone explain to me how substitution

    x = 1 / z
    dx = -dz / z^2

    works for some problems?
    He called this reciprocal substitution, as what you can literally see.
  2. jcsd
  3. Mar 20, 2016 #2


    User Avatar
    Science Advisor

    Consider the inverse gamma distribution, with pdf
    ##f(x) = \frac{\beta^\alpha}{\Gamma(\alpha)} x^{-\alpha-1} e^{-\beta / x}##.
    If you want to integrate this over say [0,a], doing the transform converts the integral into the standard gamma distribution, which is easier to work with.
  4. Apr 9, 2016 #3
    I don't see a substitution, or perhaps I don't understand your question

    What your professor did is apply the power rule, which states that if y=x^n, then y'=nx^(n-1)
    So in your particular problem this is the step-by-step way to get the derivative:
    dx/dz=(-1)z^(-1-1) ; read, the derivative of x, with respect to z equals negative one z to the power of the quantity negative one minus one.
    dx/dz= -1/z^(2) ; now multiple each side of the equation by "dz" to get rid of the dz on the left side of the equation
    dz(dx/dz)= (-1/z^(2))dz
    dx= -dz/z^(2)

    I wish I knew how to use LaTex so I could be more clear.
    In short, the notation for writing a derivative is the following
    y'(x), read "y-prime of x" is the same as saying:
    dy/dx , read "the derivative of y with respect to x"

    When we make a u substitution during integration we have to remember to replace our original "with respect to..." statement with the new "with respect to u" statement.
    du/dx=2x read, "the derivative of u with respect to x
    dx=du/2x Now substitute this back into the original integral:
    ∫(x/(x^(2)+1))dx now equals ∫(x/u2x)du
    the x in the numerator cancels with the x in the denominator so the integral is now:
    ∫(1/2u)du Now bring the 1/2 out front of the integral:
    1/2∫(1/u)du Now integrate:
    (1/2)ln(u)+C Now substitute "u" back in

    Did this answer your question?
    Last edited: Apr 9, 2016
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted