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Homework Help: Infinite line intersecting parabola only once

  1. Nov 9, 2011 #1
    I think I know the answer to this problem however I am not 100% sure.

    I must describe the conditions in which an infinite continuous line (ax+by=c) will only cross a parabola once. I believe the only answer is a straight, vertical line because while the slope of the parabola may approach zero and become extremely small it will never be zero. Therefore if the line has any slope at all, not matter how small, it will eventually cross the parabola again.

    Am I correct in this conclusion? Are there options besides a vertical line?

    Thanking you in advance,
  2. jcsd
  3. Nov 9, 2011 #2
    Should I assume since my post has many views but no replies that I am on the mark?
  4. Nov 9, 2011 #3
    Well what about a line that just touches the parabola at one point?
  5. Nov 9, 2011 #4
    I think the line must cross the parabola, not just share a point like a tangent line. I only assume this because we did tangent lines a while ago and it seems strange to have an assignment solely about finding them.
  6. Nov 9, 2011 #5


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    Homework Helper

    It is true that a line that crosses a parabola and is not tangent and which crosses it only once must be parallel to the axis of the parabola. But your proof of it is not very convincing. If there's no need to really prove it, then that's probably fine.
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