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Homework Help: The region within circles r=cosθ and r=sinθ

  1. Apr 27, 2013 #1
    1. The problem statement, all variables and given/known data
    Use a double integral to find the area of the region enclosed within both circles of r=cosθ and r=sinθ

    2. Relevant equations

    3. The attempt at a solution
    I begin by finding the region in polar co-ordinates.

    For [itex]r=\cos\theta[/itex]
    [itex]0\leq r \leq\cos\theta[/itex]


    For [itex]r=\sin\theta[/itex]
    [itex]0\leq r \leq\sin\theta[/itex]

    Now we find the region both of these have in common
    which is [itex]0\leq\theta\leq\frac{\Pi}{2}[/itex]

    For r we must find which function is the smallest and use that but [itex]\sin\theta[/itex] is greater than [itex]\cos\theta[/itex] for some portion and smaller for another so not sure what to do here.
  2. jcsd
  3. Apr 27, 2013 #2


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    Remember, r goes from r = 0 to r on the outer curve for your region. Do you see that the "outer curve" of your region is a 2 piece formula? You have to set up two integrals, unless you use some shortcut like symmetry.
  4. Apr 28, 2013 #3


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    If you were to look at a graph (these are circles) you would see that from [itex]\theta= -\pi/4[/itex] to [itex]\pi/4[/itex] r goes from 0 to [itex]sin(\theta)[/itex] while from [itex]\theta= \pi/4[/itex] to [itex]3\pi/4[/itex], r goes from 0 to [itex]cos(\theta)[/itex].
  5. Apr 29, 2013 #4
    Ahh ok I see. I wouldn't have been sure that they are symmetrical without first looking at the graph...
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