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Math Help - Perimeter of an ellipse from radii, and vice-versa

  1. #1
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    Perimeter of an ellipse from radii, and vice-versa

    Hi there,

    I'm new here. Greetings. I just posted this in the pre-Calculus forum but realize I should put it here, so....

    I need to figure out how to calculate the perimeter, or circumference, of an ellipse using the radii, and vice-versa: how to figure out the radii given the perimeter and the ratio of the two radii to each other.

    I found an equation for the perimeter, or circumference here: Circumference/Perimeter of an Ellipse: Formula(s) - Numericana)
    given the radii x and y, where x>y:

    P=3.1415*(((2*((x^2)+(y^2)))-(((x-y)^2)/2))^.5)

    The result will be an approximation but it will be close enough.

    Any chance you could help me solve this equation for x in terms of C and y, and then also for y in terms of C and x?
    Iím pretty rusty on my equations!

    Many Thanks,

    rgesh
    San Francisco, CA
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  2. #2
    MHF Contributor ebaines's Avatar
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    Re: Perimeter of an ellipse from radii, and vice-versa

    Please don't post the same question in multiple areas. This is asked and nswered here: Perimeter of an ellipse from radii, and vice-versa
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