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Math Help - Finding the coordinates of foci in an ellipse

  1. #1
    s3a
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    Finding the coordinates of foci in an ellipse

    In the following I determined the value of a to be 6 but I can't find the value of b and therefore cannot find the value of c:

    "Find the coordinates of the foci of an ellipse centred at the origin, whose vertices have the coordinates: (8, 0) and (0, -4)."

    Any help would be greatly appreciated!
    Thanks in advance!
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  2. #2
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    Hello, s3a!

    In the following I determined the value of a to be 6 . How?
    but I can't find the value of b .
    Really?
    and therefore cannot find the value of c.

    Find the coordinates of the foci of an ellipse centred at the origin,
    whose vertices have the coordinates: (8, 0) and (0, -4).
    I don't understand what you were taught . . .


    The center is at the origin: (0,0)
    . . So the vertices are at: . (\pm8,0)\,\text{ and }\,(0,\pm4)

    This tell us immediately that: . a = 8,\;b = 4

    And c is given by: . c^2 \:=\:a^2-b^2

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