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Homework Help: Finding Equation of an Orthogonal Line

  1. Nov 28, 2011 #1
    1. The problem statement, all variables and given/known data
    Let L1 be the line (0,4,5) + <1,2,-1>t and L2 be the line (-10,9,17) + <-11,3,1>t

    a) Find the line L passing through and orthogonal to L1 and L2
    b) What is the distance between L1 and L2

    3. The attempt at a solution
    I only know how to do part of part a). I can only find the direction vector of the orthogonal line by taking the cross product. I have,

    <1,2,-1> x <-11,3,1> = <5,10,25>, which I simplify to <1,2,5>.

    It doesn't seem very obvious to me how I can find a point (presumably on either L1 or L2) such that a line containing this point, pointing in the direction of <1,2,5>, passes through both L1 and L2.

    For part b), I presume that these two lines are skew. (How do you check if lines are parallel or intersect?) The distance between these two lines is

    |<5,10,25> dot [(0,4,5) - (-10,9,17)]| = |<5,10,25> dot <10,-5,-12>| = |50 - 50 - 300| = 300. So the distance is 300?
  2. jcsd
  3. Nov 28, 2011 #2
    Actually, I think I did the distance calculation wrong. What I actually have is 60/sqrt(30)
  4. Nov 28, 2011 #3


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    Looks good.

    [itex]\displaystyle \frac{60}{\sqrt{30}}=2\sqrt{30}\,[/itex]
  5. Nov 28, 2011 #4
    Okay, well it's good to know I can do part b. But what I think I really don't understand is how to do part a. One of my friends said it could be done if one has already calculated the distance, but this seems like doing the problem backwards, which I would like to avoid if possible.
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