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Determining coordinates of a point on a line perpendicular to a vector

  1. Mar 21, 2010 #1
    1. The problem statement, all variables and given/known data

    The two lines L1: r = (-1,1,0)+ s(2,1,-1) and L2: r = (2,1,2) + t(2,1,-1) are parallel but do not coincide. The point A(5,4,-3) is on L1. Determine the coordinates of a point B on L2 such that vector AB is perpendicular to L2.

    2. Relevant equations


    3. The attempt at a solution

    I'm not sure where to start on this one. I know that eventually AB dot (2,1,-1) will equal 0, but I'm not really sure what to do with this question.
  2. jcsd
  3. Mar 21, 2010 #2


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    Welcome to PF!

    Hi adrimare! Welcome to PF! :wink:
    That's right! :smile:

    So just write B in terms of t, and chug away. :wink:
  4. Mar 21, 2010 #3
    How do you do that? That's my main source of confusion here. How to write B in terms of t?
  5. Mar 21, 2010 #4


    Staff: Mentor

    Every point on L2 has to satisfy L2's equation, which means that every point on L2 has coordinates of (2 + 2t, 1 + t, 2 -t) for some value of t.
  6. Mar 21, 2010 #5
    So I do (5,4,-3)-(2+2t,2+t,2-t) to get AB and then do AB dot (2,1,-1) and find t somehow?
  7. Mar 21, 2010 #6


    Staff: Mentor

    Almost. AB = <2 + 2t, 1 + t, 2 - t> - <5, 4, -3>. The dot product of AB and <2, 1, -1> should be 0.
  8. Mar 21, 2010 #7
    Great! Thanks! I got it! I have a sort of general question for you that I don't think is worthy of its own thread really. I was just wondering what makes a line parallel to a plane?
  9. Mar 21, 2010 #8


    Staff: Mentor

    A line is parallel to a plane if it (the line) is parallel to some line segment in the plane.

    Another way to say this is that the line is parallel to the plane if it is perpendicular to the plane's normal.
  10. Mar 21, 2010 #9
    Thanks a lot! That helped with a big question! I'm starting a couple more threads with other questions if you wanna look at them.
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