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Homework Help: Tricky Integral

  1. Mar 20, 2010 #1
    Hey

    hope anybody can help me with a tricky integral ( i should check if it exists):

    [tex]\int_{0}^{\frac{\pi}{2}} \frac{1}{\sqrt{sin(x)}*cos^2(x)} dx[/tex]


    And i have really no idea where to start!

    thanks
     
  2. jcsd
  3. Mar 20, 2010 #2

    Dick

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    You should definitely check if it exists before you work too hard on it. What's the behavior like at x=0 and x=pi/2?
     
  4. Mar 20, 2010 #3

    LCKurtz

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    You aren't going to be able to find a "simple" antiderivative. Maple gives a complicated answer using Elliptic functions.
     
  5. Mar 21, 2010 #4
    So you mean, because it does not exist for Pi/2 it does not exist at all? so simple?

    And yeah, i know i did check it with maple too!
     
  6. Mar 21, 2010 #5

    vela

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    No, it's not that simple. You have to look at the behavior of the function around the singularities at x=0 and x=π/2. The integral can still converge is the function doesn't blow up too quickly around those two points.
     
  7. Mar 21, 2010 #6

    ideasrule

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    That's not necessarily true. Consider the integral of 1/sqrt(x) from x=0 to 1. The integral exists, even though 1/sqrt(x) itself does not exist at x=0.
     
  8. Mar 21, 2010 #7
    But if I try the limit x->Pi/2 i still get inf?
     
  9. Mar 21, 2010 #8

    Dick

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    The easiest way to see the problem at pi/2 is to expand your integrand in a Taylor series around x=pi/2 and realize the integrand looks like 1/y^2 where y=pi/2-x.
     
  10. Mar 21, 2010 #9
    so you mean because the taylor series at x=pi/2 --> 1/(pi/2-x)^2 which is 1/0 at pi/2 and therefore not defined the integral does not exist? if yes, can you please tell me why? thx
     
  11. Mar 21, 2010 #10

    Dick

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    As others have already said, it's not that the form is 1/0. It's that the integral of 1/y^2 in an interval including 0 doesn't not exist. Take integral from 0 to 1 of 1/y^2 and set in up as an improper integral, i.e. take integral from epsilon to 1 and let epsilon approach 0. Do you get a limit? Now do the same thing with 1/sqrt(y). Do you get a limit. They are both 1/0 AT y=0.
     
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