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Math Help - Double integral with polar coordinates

  1. #1
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    Double integral with polar coordinates

    Evaluate the given integral by changing to polar coordinates.



    ...where D is the region bounded by the semicircle and the y-axis.

    ----

    I do all my scrap work on a tablet, so here's what I came up with. I did all of the integration in WolframAlpha just to make sure it was correct (did it manually the first three times). What am I doing wrong?



    Any help or hints would be appreciated!

    Thanks.
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  2. #2
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    The limits of integration are incorrect. Theta should be from -pi/2 to pi/2 as the region of integration is bounded by x = sqrt(49-y^2) and x = 0 (i.e. the right hemicircle of radius 7).

    EDIT:

    Your integration is also incorrect.

    Separating the integration with respect to theta from r, we obtain immediately theta evaluated from -pi/2 to pi/2 which is equal to pi. Evaluating the integration with respect to r, not forgetting the Jacobian factor r, we obtain after the substitution u = -r^2, change of limits (including reversal due to the introduced negative sign): (pi)(1/2)(e^u) | (u1 = -49, u2 = 0) which gives (pi/2)(1-e^-49).
    Last edited by TaylorM0192; April 19th 2011 at 07:27 PM.
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  3. #3
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    Your work looks fine to me.

    The limits of integration are not quite right as pointed out by TaylorM0192.

    The integration over r looks fine to me.
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  4. #4
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    Thanks, both of you, for your assistance! From your advice I used all of my previous work but just changed the limits of integration on theta to (-pi/2) to (pi/2) and it worked like a charm.

    Cheers!
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