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Yet another vector proof

  1. Oct 20, 2012 #1
    1. The problem statement, all variables and given/known data
    If [itex] (a \times b) \times (c \times d) = 0 [/itex], show that a,b,c,d are coplanar.

    2. Relevant equations

    3. The attempt at a solution
    I have done the converse of this problem, but am having trouble on how to do this. Can we perhaps use Lagrange's identity. How can I start?

  2. jcsd
  3. Oct 21, 2012 #2


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    It's not true.

    If [itex](a \times b)=0\,,[/itex] the c & d can be anything.
  4. Oct 21, 2012 #3
    Thanks Sammy!

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