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  1. Aug 14, 2004 #1
    If you had line AB is parallel to BC and BC is parallel to CD, is AB parallel to CD?
    ----> Not if AB=CD since a line (at least in Euclidean Geometry) cannot be parallel to itself.
    How would you prove that AB is not line CD?

    PLEASE NOTE: Base all your input in the realm of Euclidean geometry.
  2. jcsd
  3. Aug 14, 2004 #2


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    If you refer to the line AB as the line going through two (different) given points, point A and point B, then clearly AB and BC both pass through the same point, point B.
    Therefore if AB is not equal to BC, they will not be parallel.

    Same argument goes for BC and CD.
  4. Aug 14, 2004 #3


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    BTW, the definition of "parallel" with which I'm familiar says that a line is parallel to itself.
  5. Aug 16, 2004 #4
    No, the definition of parallel lines is: Two lines are parallel if and only if they do not intersect and they are not in the same plane. This definition holds in Euclidean geometry.
    Let line AB and CD be parallel lines. Let AB=CD. Then C=A and B=D. The two lines have points in common, therefore they intersect. By definition, parallel lines do not intersect. Therefore a line cannot be parallel to itself.

    All of this I base on the laws of Euclidean Geometry.
  6. Aug 16, 2004 #5

    How about lines without points in common?
  7. Aug 16, 2004 #6
    "AB is parallel to BC"

    If both lines go trough a point B, they have at least one point in common. Therefore, they can't be parallel (that is, if a line cannot be parallel to itself).
  8. Aug 16, 2004 #7
    I agree with you, koroljov.

    now let us consider AC and GP and VL. Prove that AC and VL are two different lines and they are both parallel to GP.
  9. Aug 16, 2004 #8


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    Please be more careful in quoting definitions. That definition is completely wrong!

    I would also point out that if AB and CD are lines such that AB= CD, it does NOT follow that "C= A and B= D". That does not follow even for line segments rather than lines.
  10. Aug 16, 2004 #9
    I've always believed two parallel lines could be AT THE SAME PLANE...
  11. Aug 16, 2004 #10
    yeah "is parallel to" is even equivalence relation:
    1. AB || AB
    2. if AB || BC then BC || AB
    3. if AB || BC & BC || CD then AB || CD
    what's all the fuss about everybody? a line can always be parallel to itself
  12. Aug 16, 2004 #11


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    I always thought that to be parrallel you only needed the same slope in two lines.

    Also, when he said AB = CD, I don't think he was multiplying, instead he was saying the lines have the same end points. A = C and B = D.

    So... is x^2 'parrallel' to x^2+1?
  13. Aug 17, 2004 #12


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    Yes, that's true but that statement, requiring trigonometry, is too complicated to be considered a definition.

    Yes, everyone understood that. My point was: first LINES don't have endpoints, line SEGMENTS do (and we were talking about "parallel" lines which doesn't apply to segments). Also, even for line segments AB, CD, in which A, B, C, D ARE the endpoints, AB= CD does not necessarily imply A= C and B= D. It might be that A= D and B= C!

    No, the concept of parallel we are talking about here (Euclidean geometry) only applies to straight lines.
    Certainly, you could extend the concept of parallel (in several different ways, possibly) to curves. I suspect that in any reasonable definition of "parallel" for curves, the graphs given by y= x<sup>2</sup> and y= x<sup>2</sup>+ 1 would be parallel.
  14. Aug 18, 2004 #13
    According to this source:

    Parallel lines are defined as follows:

    Parallel straight lines are straight lines which, being in the same plane and being produced indefinitely in both directions, do not meet one another in either direction.

    How is that different from what I stated? (I am inquiring with genuine curiosity, not to be defensive)
  15. Aug 18, 2004 #14
    I'm just throwing something out here, but regarding "parallel" curves (of which I've never heard any definition) such as x^2 and x^2 + 1, I would imagine that this could be solved by use of derivatives. Since the derivative is 2x in both cases, then they could be considered parallel.

    However, since no actual definition exists, you could theoretically create any critieria you wish. ;)
  16. Aug 19, 2004 #15
    If the first curve were sin x and the second one were sin (x + 5) + 5, you'd probably say they were parallel even though their derivatives are different. You tend to say two things are "parallel" when one can be transformed into the other solely by translation.
  17. Aug 20, 2004 #16


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    Yes, but your original post was (I just "cut and paste" it):

    "No, the definition of parallel lines is: Two lines are parallel if and only if they do not intersect and they are not in the same plane. This definition holds in Euclidean geometry."

    Specifically, you said "they are not in the same plane" which is not what you intended.
    Last edited by a moderator: Apr 21, 2017
  18. Aug 28, 2004 #17
    oh, thanks!!

    I still don't understand how only one line, A can be parallel to itself.
  19. Aug 29, 2004 #18


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    Depends entirely on the specific definition. I, personally, would be inclined to say that two lines are parallel if and only if they have no point in common so that a line is not parallel to itself, but if you are focusing on direction (two lines are parallel if and only if they have the same direction) you could certainly say a line is parallel to itself. It's not a definition I would use but I wouldn't argue with Hurkyl's use of it.
  20. Sep 4, 2004 #19
    If that's true, then skew lines are parallel.

    But that isn't the definition given by Euclid. I am focusing on Euclid's definition.

    Anyway, saying that a line is parallel to itself does not satisfy the hypothesis of the definition (the second one) that you gave. The hypothesis says "two lines are parallel if...", but if a line is parallel to itself, then there is no need for a second line right? I'm probably mistaken though.

    I don't challenge Hurkyl (among others including you HallsofIvy); I just ask how he arrives at conlusions or just ask him questions. I am doing the same here.
    Last edited: Sep 4, 2004
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