Calculating the area between a petal and a circle (Polar Coords)

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Yes, the curves intersect at $\theta = \pm \frac \pi 9$ and you can use symmetry to integrate from $0$ to the maximum value of $\theta$, then double the result. What you have missed is that the green area extends above the line $\theta=\frac \pi 9$. From $\theta=0$ to $\theta=\frac \pi 9$ you are right, the right curve is $r=2\cos (3 \theta)$ and the left curve is $r=1$. There is some area at $\theta$ larger than $\frac \pi 9$ where the left limit is $r=0$ and the right limit is $r=2\cos (3 \theta)$

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fcvuyvuyv
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Updated on August 01, 2022

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  • fcvuyvuyv
    fcvuyvuyv over 1 year

    enter image description here

    The question states: The graph of $2\cos(3\theta)$ has three petals (also called "leaves" or "lobes"). The intersection of one of those petals with the circle $r = 1$ is shaded in the figure. Find the area of the shaded region.

    I already know the formula, $$A = 1/2\int_a^b f(\theta)^{2}-g(\theta)^2d\theta$$ the problem obviously is finding the bounds. I set the functions equal to each other and solved:

    $2\cos(3x) = 1$

    $\cos(3x) = 1/2$

    $3x = \arccos(1/2)$

    $3x = \pi/3$

    $x = \pi/9$

    So wouldn't the functions intersect when $\theta = -\pi/9, \pi/9$?

    Can I not just exploit symmetry and calculate the integral from $0$ to $\pi/9$ then multiply the entire thing by $2$?

    And Finally, would $r = 1$ be the right most curve, and the $2\cos(3\theta)$ be the left most?

    • Matthew Leingang
      Matthew Leingang over 7 years
      Yes to all your questions.
  • fcvuyvuyv
    fcvuyvuyv over 7 years
    Thanks for the answers to my questions but I'm still confused on how to then find the proper bounds to set up the integral. I'm not sure I grasp the concept.
  • Ross Millikan
    Ross Millikan over 7 years
    The upper bound is the angle at which the curve approaches $r=0$. For example, at $\theta=\frac \pi 8 \gt \frac \pi 9$, we have $r =\sqrt{2-\sqrt 2} \approx 0.765$ so there is area you will miss with an upper limit of $\frac \pi 9$