Two lines intersect at three points?

  1. Two lines intersect at three points???

    I have a question that's been bugging my mind. :confused: Imagine the lines y=x and y=2x. Let's find their intersection point.
    y=x and
    y=2x so that
    x=2x If we give x the value of 0 the equation will be correct. (1st intersection)
    But if we give x the value of infinity the equation will be correct once again.
    (inf=2*inf) (2nd intersection)
    If we put negative infinity for x the equation will be correct. (3rd equation)
    We have found three intersection points. We know that lines either do not intersect or intersect at only one point or intersect at infinite points. They certainly cannot intersect at three points. Is it because infinity is not a real number, which means that we don't care about the points of intersection whose coordinates are equal to either infinity or negative infinity? :wink:
  2. jcsd
  3. Zurtex

    Zurtex 1,123
    Science Advisor
    Homework Helper

    I assume x and y are real numbers? In which case you can not substitute in infinity because it is not a real number.
  4. NateTG

    NateTG 2,514
    Science Advisor
    Homework Helper

    In Euclidean (planar) geometry, there are no points at infinity - so the two lines only meet at x=y=0.

    There are other geometries where lines can meet in more than one place. For example, spherical or hyperbolic geometry.
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