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Math Help - [SOLVED] Directrix and Focus

  1. #1
    Member looi76's Avatar
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    [SOLVED] Directrix and Focus

    If the Directrix is negative then the Focus will have a positive sign?

    Example:

    Directrix: x=-5 then Focus (5,0)

    Am I right, or the sign remains the same?
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  2. #2
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    This doesn't seem to always be true. The focus and directrix are equidistant from the vertex when connecting the three in a line. Consider a basic case where the standard parabola of y=x^2 was vertically shifted up 30 units. The focus is slightly above the vertex and the directrix is a horizontal line running slightly below the vertex but both are always positive.
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  3. #3
    Member looi76's Avatar
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    I forgot to mention that the Vertex is (0,0). So, if the Vertex is at the Origin. Does this rule apply?
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  4. #4
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    Quote Originally Posted by looi76 View Post
    I forgot to mention that the Vertex is (0,0). So, if the Vertex is at the Origin. Does this rule apply?
    Yes it seems so to me, just remember one is a point and the other a line.
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