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Tangent plane, why is it orthongonal and not parallel?

  1. Aug 3, 2011 #1
    1. The problem statement, all variables and given/known data


    Find equation of the plane

    The plane though P(6,3,2) and is perpendicular to vector <-2,1,5>

    Why would it be -2(x - 6) + (y - 3) + 5(z - 2) = 0?

    If it is perpendicular to <-2,1,5>, shouldn't be the cross product of some other vector with this? Using <-2,1,5> wouldn't mean it is parallel to the vector instead?



    3. The attempt at a solution
     
  2. jcsd
  3. Aug 3, 2011 #2

    Dick

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    Are you saying the CROSS product of two perpendicular vectors is zero? If not what are you saying?
     
  4. Aug 3, 2011 #3
    I am saying that

    [PLAIN]http://img34.imageshack.us/img34/9647/unledseh.jpg [Broken]

    A plane's direction is determined by it's normal vector right? So if it is perpendicular to the vector <2,-1,5>, why would its normal vector be the same as <2,-1,5>?
     
    Last edited by a moderator: May 5, 2017
  5. Aug 3, 2011 #4

    Dick

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    Perpendicular means the same thing as normal, as far as I know. If the plane is perpendicular to <2,-1,5> then it's also normal to <2,-1,5>. I'm really not sure what your question is. The normal vector is not parallel to the plane if that's what confusing you. Describing it as the 'direction of the plane' doesn't mean it's parallel to the plane.
     
    Last edited by a moderator: May 5, 2017
  6. Aug 3, 2011 #5
    But <2,-1,5> is just a scalar multiple of <2,-1,5> (scalar = 1), that is parallel?
     
  7. Aug 3, 2011 #6

    Dick

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    I may have figured out what is confusing you. A plane doesn't have a unique parallel direction. So the phrase 'direction of the plane' means its normal. Not its parallel. See my previous post.
     
  8. Aug 4, 2011 #7
    Is it because of this?

    [PLAIN]http://img841.imageshack.us/img841/6863/unleditl.jpg [Broken]
     
    Last edited by a moderator: May 5, 2017
  9. Aug 4, 2011 #8

    HallsofIvy

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    I have no clue what that is supposed to represent. However, I do note that you asked about a plane and a vector but your pictures show only two vectors, no plane.
     
  10. Aug 4, 2011 #9
    Oh, that is supposed to be a plane and the two vectors lie on the plane
     
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