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Math Help - Equation for an ellipse

  1. #1
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    Equation for an ellipse

    Give the equation for the ellipse with foci at (0, +/-2) and semi-major axis 3.

    I find that:

    a = 3
    b = sqrt(5)
    c = +/- 2

    Thus by the standard equation of the ellipse:

    x^2/a^2 + y^2/b^2 = 1

    I get:

    x^2/9 + y^2/5 = 1

    However, the correct answer is supposed to be:

    x^2/5 + y^2/9 = 1

    What is wrong with my approach?
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  3. #3
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    Quote Originally Posted by posix_memalign View Post
    Give the equation for the ellipse with foci at (0, +/-2) and semi-major axis 3.

    I find that:

    a = 3
    b = sqrt(5)
    c = +/- 2

    Thus by the standard equation of the ellipse:

    x^2/a^2 + y^2/b^2 = 1

    I get:

    x^2/9 + y^2/5 = 1

    However, the correct answer is supposed to be:

    x^2/5 + y^2/9 = 1

    What is wrong with my approach?
    note that the foci and major axis are on the y-axis, not the x-axis.
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