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Homework Help: Equation of a line perpendicular to 2 vectors

  1. Apr 10, 2008 #1
    1. The problem statement, all variables and given/known data

    Let L be a line in R3 passing through(-1,1,2) and is perpendicular to vectors V1 (-1,1,-1)
    and V2 (1,1,1). Find an equation for L in parametric form.

    2. Relevant equations

    3. The attempt at a solution
    using vector V2
    A possible equation for L=>(x,y,z)=(-1,1,2)+t(1,1,1)

    I don't know if I am right.
  2. jcsd
  3. Apr 10, 2008 #2


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    The line you give is not perpendicular to V2. Any point on (-1,1,2)+ t(1,1,1) has position vector (t-1, t+1, t+ 2) and its dot product with (1, 1, 1) is (t-1)+ (t+1)+ (t+2)= 3t+2, not 0. I'm not sure why you think that would give you a line perpendicular to (1,1,1).

    But even if you were to find a line perpendicular to V2, there is no reason to think that line would also be perpendicular to V1! You need a direction vector that is perpendicular to both. Do you know how to calculate a vector perpendicular to both V1 and V2?
  4. Apr 10, 2008 #3
    To be honest, I don't know how to find a vector perpendicular to both V1 and V2.
    I can find 2 equations of a lines perpendicular to each of the vectors
    (1)=> (x,y,z)=(-1,1,2)+t(-1,1,1)
    (2)=> (x,y,z)=(-1,1,2)+t(1,1,1)
    now where do I go from here.
  5. Apr 10, 2008 #4


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    Science Advisor

    Do you know what the cross product of two vectors is?
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