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Homework Help: Vector multiplication

  1. Aug 23, 2014 #1
    1. The problem statement, all variables and given/known data
    Still having a little trouble so here's the problem.

    (ex + ez) x (3ey - 4ez)

    3. The attempt at a solution

    (ex * ez) + (ex * (-4ez)) + (ez * 3ey) + ( ez * (-4ez)

    now, these are all orthogonal to each other, so, for example, if I have ex * ey then I should end up with ez, shouldn't I?
    So here is my solution.

    = 3ez - 4ey + 3ex - 4ez
    = 3ex - 4 ey + ez

    The solution shown to be correct is -3ex + 4ey + 3ez

    so what have I done wrong, please?
  2. jcsd
  3. Aug 23, 2014 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I assume by * you mean X (vector product). So yes, ##e_x \times e_y = e_z##. What about ##e_y \times e_x##, ##e_x \times e_z##, and the other combinations? (You are making a sign error.)
  4. Aug 23, 2014 #3
    Thank you.
    A simple thing becomes clear again.
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