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Math Help - Cutting a Semicircle into 2 Parts

  1. #1
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    Cutting a Semicircle into 2 Parts

    How do you solve this?
    Last edited by yeah:); September 29th 2010 at 08:32 AM.
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  2. #2
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    The horizontal line will create a segment; go here:
    Circle Sector and Segment
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  3. #3
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    Not sure if this is the simplest way but you require half the area of a semi circle to be \frac{1}{4}\times \pi\times r^2 to equal the segment part of \frac{1}{2}\times (\theta -\sin \theta) r^2 giving

    \frac{1}{4}\times \pi\times r^2=\frac{1}{2}\times (\theta -\sin \theta) r^2

    \frac{1}{4}\times \pi=\frac{1}{2}\times (\theta -\sin \theta)

    \frac{\pi}{2}= \theta -\sin \theta
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    Quote Originally Posted by pickslides View Post
    Not sure if this is the simplest way but you require half the area of a semi circle to be \frac{1}{4}\times \pi\times r^2 to equal the segment part of \frac{1}{2}\times (\theta -\sin \theta) r^2 giving

    \frac{1}{4}\times \pi\times r^2=\frac{1}{2}\times (\theta -\sin \theta) r^2

    \frac{1}{4}\times \pi=\frac{1}{2}\times (\theta -\sin \theta)

    \frac{\pi}{2}= \theta -\sin \theta
    What is the most simple way of going from this equation (i.e. what is the easiest way of solving it, obtaining an accurate, numerical value)? How do you then find what the problem is looking for?
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  5. #5
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    What is the most simple way of going from this equation (i.e. what is the easiest way of solving it, obtaining an accurate, numerical value)?
    Incidentally, there is a similar question in the Trigonometry forum, but it does not offer a precise solution (yet).
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  6. #6
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    Quote Originally Posted by emakarov View Post
    Incidentally, there is a similar question in the Trigonometry forum, but it does not offer a precise solution (yet).
    So, would anyone be able to propose a clever method of solving this problem?
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  7. #7
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    Quote Originally Posted by yeah:) View Post
    So, would anyone be able to propose a clever method of solving this problem?
    We are solving for the angle \theta which can be done numerically. Do this using a spreadsheet or graphical calculator.
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  8. #8
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    It is not clear to me what the phrase "find... the line" means in the original question. It is possible that the distance of the bisecting line from the diameter is a nice number expression built from natural numbers, \pi, the four arithmetic operations, radicals, and trigonometric functions. However, there may not be such an expression since the number of reals is vastly greater than the number of expressions (even though both are infinite). So, sometimes the description "the number x such that x-\sin x=\pi/2" is as good as one can get. I would ask the instructor to clarify what kind of answer is expected. I believe that in this case it is not something like asking for a hint.
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  9. #9
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    Agreed, it seems more practical to ask what is the reduction in the radius such that the semicircle is half the area given the horizontal cut.
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  10. #10
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    Quote Originally Posted by pickslides View Post
    Agreed, it seems more practical to ask what is the reduction in the radius such that the semicircle is half the area given the horizontal cut.
    The answer expected is the point down the distance of the radius of the circle, through which it would be possible to draw a horizontal line, in order to cut the semicircle into 2 parts exactly. What is the most simple way of obtaining an accurate numerical value for this point?
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  11. #11
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    This is answered in post #7.

    You just need to then find the relationship between r and \theta
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  12. #12
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    Quote Originally Posted by yeah:) View Post
    What is the most simple way of obtaining an accurate numerical value for this point?
    Define "simple". "Simple" to you may be "complicated" to me ... or the other way around!
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  13. #13
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    Quote Originally Posted by Wilmer View Post
    Define "simple". "Simple" to you may be "complicated" to me ... or the other way around!
    The main thing is that you explain it well to me - please show me, with explanation, how you would solve this problem.
    Last edited by yeah:); September 29th 2010 at 08:33 AM.
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  14. #14
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    Quote Originally Posted by yeah:) View Post
    The main thing is that you explain it well to me - please show me, with explanation, how you would solve this problem. For example, how would you do it with the Newton method, or any other iteration?
    You have been given the required equation and shown where it comes from. You have been told to take a numerical approach in solving the equation (because an exact solution is not possible). As far as I can see, the question you originally asked has been completely answered.

    It is up to you now to decide what to do. What numerical approaches have you been taught? Do you know how to use technology (such as a graphics calculator) to solve an equation? These issues (that were not flagged in the original post) do not belong in the Geometry subforum.
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  15. #15
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    OK - thank you very much!
    Last edited by yeah:); September 29th 2010 at 08:34 AM.
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