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Two curves in a cut point would have a same tangent

  1. Sep 16, 2010 #1
    Ola,

    If we have two curves, and they cut them selves in a way like these two: y=x^2 and
    x^2+(y-1)^2=1. Does it always mean that those two curves in a cut point would have a same tangent, in other words do they need to have the same derivative in that spot?

    Thanks!!
     
  2. jcsd
  3. Sep 16, 2010 #2

    mathman

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    Re: Tangent

    Your question is confusing - what has "cut" to do with anything? However the two curves are tangent to each other at the origin, so the tangent is the same for both at this point.
     
  4. Sep 17, 2010 #3
    Re: Tangent

    Well it is little confusing. But I do not know what other word to use instead "cut" maybe
    carve, or bisection or contact. And instead "them selves" use one another.

    Yes, these two are tangent in point (0,0), but I want to ask is that case with every two curves that contact in the way like in the example that I wrote.

    Another example of contact that I am talking about is: y=x^2+x, and y=x^3+x in (0,0) point.
    And the contact that I am not talking about is: y=x^2+x, and y=x^3+x in (1,2) point.
     
  5. Sep 17, 2010 #4

    HallsofIvy

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    Re: Tangent

    "itersect" would be better than "cut". And, yes, they do not intersect themselves, they intersect each other. No, the fact that two curves intersect does not mean they have the same tangent there (which was what you asked). Two intersecting straight lines specifically do NOT have the same tangent at the point of intersection. That's so obvious, it's probably not what you were asking.

    If you meant to ask "if two curves intersect like these curves do at (0, 0) do they have the same tangent" then, (1) you did not tell us in your original post that you only talking about the intersection at (0, 0), (2) you will need to specify what you mean by "like". The curves you give are tangent to one another at (0,0) so, again obviously, if two curves are tangent to one another at a point where they intersect then, yes, they have the same tangent there. That is precisely what "tangent to one another" means!
     
  6. Sep 17, 2010 #5
    Re: Tangent

    it doesnt have same tangent always.
     
  7. Sep 17, 2010 #6
    Re: Tangent

    @Little ant

    Could you please give me some proof or an example of that fact?

    thanks!
     
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