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Vector Cross Product Proof

  1. Jan 5, 2013 #1

    Jow

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    If e1 and e2 are vectors in ℝ3 show that e1 x e2 = e3, e2 x e3 = e1 and e3 x e1 = e2. I have tried to prove this but I can't get it.

    My attempt:
    Step 1: [a1, a2, a3] x [b1, b2, b3] = [a2b3-a3b2, a3b1-a1b3, a1b2-a2b1]
    Step 2: [b1, b2, b3] x [a2b3-b2a3, a3b1-a1b3, a1b2-a2b1] = [b2(a1b2-a2b1)-b3(a3b1-a1b3), b3(a2b3-a3b2)-b1(a1b2-a2b1), b1(a3b1-a1b3)-b2(a2b3-b2a3)] ... nothing cancels and I do not end up with [a1, a2, a3], which I should shouldn't I?
    I also try this with actual numbers but it still doesn't work. Am I doing something completely wrong? I understand, geometrically, why this should happen because the cross product is orthogonal to the two vectors. Am I doing something wrong or is my text book wrong? (I am pretty sure the former is the right answer to that question).
     
  2. jcsd
  3. Jan 5, 2013 #2

    Dick

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    I think they mean e1=[1,0,0], e2=[0,1,0] and e3=[0,0,1]. They are special vectors. It's not true for any three vectors.
     
  4. Jan 5, 2013 #3

    Jow

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    Oh good. I thought I was missing something really important. I must have just misread the question. Thanks.
     
  5. Jan 6, 2013 #4

    HallsofIvy

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    In fact, one way of defining the "cross product" is to assert that [itex]e_1\times e_2= e_3[/itex], [itex]e_2\times e_3= e_1[/itex], [itex]e_3\times e_1= e_2[/itex], that the product is "bilinear" and anti-commutative.
     
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