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Thread: 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 $\displaystyle a$ to be 6 . How?
    but I can't find the value of $\displaystyle b$ .
    Really?
    and therefore cannot find the value of $\displaystyle c.$

    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: $\displaystyle (0,0)$
    . . So the vertices are at: .$\displaystyle (\pm8,0)\,\text{ and }\,(0,\pm4)$

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

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

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