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Math Help - Negative radius

  1. #1
    Forum Admin topsquark's Avatar
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    Negative radius

    I have just been informed (and been able to verify on the web) that the polar coordinate system allows the use of r < 0.

    I was never taught that. When did this convention start being used? Did my teachers simply not happen to use it?

    -Dan
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  2. #2
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    Quote Originally Posted by topsquark View Post
    I have just been informed (and been able to verify on the web) that the polar coordinate system allows the use of r < 0.

    I was never taught that. When did this convention start being used? Did my teachers simply not happen to use it?

    -Dan
    "The r is normally limited to non-negative numbers, but negative r is sometimes used to represent a point at an angle pi apart from the theta specified."

    from polar coordinates@Everything2.com
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  3. #3
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    Quote Originally Posted by topsquark View Post
    I have just been informed (and been able to verify on the web) that the polar coordinate system allows the use of r < 0.

    I was never taught that. When did this convention start being used? Did my teachers simply not happen to use it?

    -Dan
    You need the idea of a negative value for r when drawing graphs of things like r = 1 - 2 \sin \theta. For this example, when \theta = \frac{\pi}{2} you get r = -1. The corresponding Cartesian coordinates of [-1, pi/2] are (0, -1).
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