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Homework Help: Cross product for parallel vectors

  1. Mar 13, 2010 #1
    1. The problem statement, all variables and given/known data

    Is the line through (4,1,-1) and (2,5,3) parallel to the line through (-3,2,0) and (5,1,4)?

    2. Relevant equations

    3. The attempt at a solution

    Line one 'direction' = (-2,4,4) = A
    Line two 'direction' = (8,-1,4) = B

    I remember that the cross product of two vectors is zero if they are parallel, but AxB is not the zero vector; the answer in the book says they are indeed parallel...

    Is this not the right method?

    Adrian ^^
  2. jcsd
  3. Mar 13, 2010 #2


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    Homework Helper

    If they were parallel, you could write one direction as a scalar multiple of the other. Since you cannot do that as well as the cross-product is not zero, the vectors are not parallel.
  4. Mar 16, 2010 #3
    Cheers, thanks for that. I got it all figured out now..

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