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Math Help - This looks like a crazy integral...please help

  1. #1
    s3a
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    This looks like a crazy integral...please help

    How do I evaluate: http://www.wolframalpha.com/input/?i=integral+sqrt((x^2+%2B+1)^2%2F(x^2+-+1)^2)%2C+from+0+to+1%2F2 ?

    Any help would be greatly appreciated!
    Thanks in advance!
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  2. #2
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    Quote Originally Posted by s3a View Post
    How do I evaluate: http://www.wolframalpha.com/input/?i=integral+sqrt((x^2+%2B+1)^2%2F(x^2+-+1)^2)%2C+from+0+to+1%2F2 ?

    Any help would be greatly appreciated!
    Thanks in advance!
    \sqrt{\frac{(x^2 + 1)^2}{(x^2 - 1)^2}} = \frac{x^2 + 1}{x^2 - 1}

     = \frac{x^2 - 1 + 2}{x^2 - 1}

     = 1 + \frac{2}{x^2 - 1}

     = 1 + \frac{2}{(x - 1)(x + 1)}.


    Now by using partial fractions:

    \frac{A}{x - 1} + \frac{B}{x + 1} = \frac{2}{(x - 1)(x + 1)}.

    LHS: \frac{A}{x - 1} + \frac{B}{x + 1} = \frac{A(x + 1)}{(x - 1)(x + 1)} + \frac{B(x - 1)}{(x - 1)(x + 1)}

     = \frac{A(x + 1) + B(x - 1)}{(x - 1)(x + 1)}.


    The numerators are equal, so

    A(x + 1) + B(x - 1) = 2

    Ax + A + Bx - B = 2

    (A + B)x + A - B = 0x + 2.


    So A + B = 0 and A - B = 2.

    Therefore A = B + 2


    B + 2 + B = 0

    2B + 2 = 0

    B = -1.


    A = B + 2

    A = -1 + 2

    A = 1.


    Therefore \frac{2}{(x - 1)(x + 1)} = \frac{1}{x - 1} - \frac{1}{x + 1}.



    So your integral:

    \int{\sqrt{\frac{(x^2 + 1)^2}{(x^2 - 1)^2}}\,dx} = \int{1 + \frac{1}{x - 1} - \frac{1}{x + 1}\,dx}.

    You should be able to go from here.
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  3. #3
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    Quote Originally Posted by Prove It View Post
    \sqrt{\frac{(x^2 + 1)^2}{(x^2 - 1)^2}} = \frac{x^2 + 1}{x^2 - 1}
    Be careful here. Since we are integrating from 0 to 1/2, we have that x^2-1<0, so we should actually get:

    \sqrt{\frac{(x^2 + 1)^2}{(x^2 - 1)^2}} = - \frac{x^2 + 1}{x^2 - 1}

    This doesn't invalidate the rest of the work, but we just need to switch the sign.

    In other words, we should instead use:

    \int_0^{1/2} \sqrt{\frac{(x^2 + 1)^2}{(x^2 - 1)^2}}\,dx ~=~ -\int_0^{1/2} \left( 1 + \frac{1}{x - 1} - \frac{1}{x + 1} \right) dx
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  4. #4
    s3a
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    Where does the 1 come from in the integral? (after the square root step)
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  5. #5
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    read above, quite obvious from the previous work shown.

    and it's worth to mention that \frac{2}{(x-1)(x+1)}=\frac{(x+1)-(x-1)}{(x-1)(x+1)}=\frac{1}{x-1}-\frac{1}{x+1}.
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  6. #6
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    Quote Originally Posted by s3a View Post
    Where does the 1 come from in the integral? (after the square root step)
    When using partial fraction decomposition, you need the numerator to have lower degree than the denominator. In a more complex problem you may have to use polynomial long division to see the reduction, but in this case it's pretty easy to rewrite the expression to get it.

    \frac{x^2 +1}{x^2 - 1} = \frac{x^2 - 1 + 2}{x^2 - 1} = \frac{x^2-1}{x^2-1} + \frac{2}{x^2-1} = 1 + \frac{2}{x^2-1}
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  7. #7
    s3a
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    I still fail to see where the 1 came from. Here is my new work (attached).
    Attached Thumbnails Attached Thumbnails This looks like a crazy integral...please help-mywork.jpg  
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  8. #8
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    Quote Originally Posted by s3a View Post
    I still fail to see where the 1 came from. Here is my new work (attached).
    You attempted to use partial fraction decomposition on the following fraction

    \frac{-(x^2+1)}{x^2-1}

    You cannot do that! Partial fraction decomposition cannot be used on this fraction because the degree of the numerator is not less than the degree of the denominator.

    Notice how you got

    \frac{1}{x+1} - \frac{1}{x-1}

    when you did the work. But now, let's check the answer:

    \frac{1}{x+1} - \frac{1}{x-1} = \frac{1}{x+1} \left( \frac{x-1}{x-1} \right) - \frac{1}{x-1} \left( \frac{x+1}{x+1} \right)

    = \frac{x-1}{(x+1)(x-1)} - \frac{x+1}{(x-1)(x+1)}= \frac{x-1-(x+1)}{x^2-1} = \frac{-2}{x^2-1}

    But this doesn't equal the original fraction.
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  9. #9
    s3a
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    I forgot to check the degree of the exponents. After having done long division, etc, I get the right answer! Thanks everyone!
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