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Integration w/ trig. substitution (x^2 + 1) / (x^2 - 2x + 2)^2

  1. Oct 4, 2009 #1
    Another even numbered problem in my book, so no textbook answer. I checked it in WolframAlpha(WA), but the answer came out slightly different. Hopefully no typos in this writeup.

    1. The problem statement, all variables and given/known data
    [tex]\int \frac{x^2 + 1}{(x^2 - 2x + 2)^2}dx[/tex]


    2. Relevant equations
    I factor the denominator:
    [tex](x^2 - 2x + 2) = (x - 1)^2 + 1[/tex]

    Some potential trig. substitution:
    [tex]tan(\Theta) = x - 1[/tex]
    [tex]x = tan(\Theta) + 1[/tex]
    [tex]dx = sec^2(\Theta)d\Theta[/tex]
    [tex](x - 1)^2 + 1 = sec^2(\Theta)[/tex]


    3. The attempt at a solution
    [tex]= \int \frac{x^2 + 1}{((x - 1)^2 + 1)^2}dx[/tex]

    [tex]= \int \frac{(tan^2(\Theta) + 2tan(\Theta) + 1) + 1}{(sec^2(\Theta))^2} sec^2(\Theta)d\Theta[/tex]

    [tex]= \int sin^2(\Theta)d\Theta + 2\int sin(\Theta)cos(\Theta)d\Theta + 2\int cos^2(\Theta)d\Theta[/tex]

    [tex]= \frac{1}{2}\int (1 - cos(2\Theta))d\Theta + 2 \int sin(\Theta)cos(\Theta)d\Theta + 2 \int 1/2(1 + cos(2\Theta))d\Theta[/tex]

    [tex]= \frac{1}{2}\Theta - \frac{1}{4}sin(2\Theta) + sin^2(\Theta) + \Theta + \frac{1}{2}sin(2\Theta) + C[/tex]

    [tex]= \frac{3}{2}arctan(x-1) + \frac{1}{4}2sin(\Theta)cos(\Theta) + sin^2(\Theta) + C[/tex]

    [tex]= \frac{3}{2}arctan(x-1) + (\frac{1}{2})(\frac{x-1}{\sqrt{x^2-2x+2}})(\frac{1}{\sqrt{x^2-2x+2}}) + \frac{(x-1)^2}{x^2-2x+2} + C[/tex]

    [tex]= \frac{3}{2}arctan(x-1) + \frac{2x^2-3x+1}{2(x^2-2x+2)} + C[/tex]

    However, plugging it into WA, the answer it spits out is:

    [tex]= -\frac{3}{2}arctan(1-x) + \frac{x-3}{2(x^2-2x+2)} + C[/tex]

    I get the 1st term is the same thing, but in the 2nd term, I ended up with 2x^2-3x+1 instead of just x-3. Anyone see a misstep?

    I looked through the steps of WA, but I'm not really familiar with what it's saying.
     
    Last edited: Oct 4, 2009
  2. jcsd
  3. Oct 5, 2009 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi SpicyPepper! :smile:
    No, the book's C is your C plus one …

    always adjust the constant of integration so as to make any fraction a "proper" one. :wink:
     
  4. Oct 5, 2009 #3
    why is it always so obvious after the fact :p

    thx
     
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