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Homework Help: Vector Orthogonality proof

  1. Sep 4, 2012 #1
    1. The problem statement, all variables and given/known data

    Show that

    ||b||a + ||a||b and ||b||a - ||a||b are orthogonal vectors.

    2. Relevant equations

    3. The attempt at a solution

    After analyzing it and trying to prove it to no avail, I don't even think it's a true statement.
  2. jcsd
  3. Sep 4, 2012 #2


    Staff: Mentor

    If two vectors are othogonal, their dot product will be zero. What do you get if you dot the two vectors in this problem?
  4. Sep 4, 2012 #3
    Nevermind, careless error.
  5. Sep 4, 2012 #4
    Though I was able to prove it, I do find it difficult to wrap my head around the fact that what makes it work is the scalar multiplication..
  6. Sep 4, 2012 #5


    Staff: Mentor

    That's probably the most important use of the dot product - determining whether two vectors are orthogonal. Less important, IMO, is the ability to find the angle between two vectors.
  7. Sep 4, 2012 #6
    I mean the scalar multiplied by vector multiplication. The dot product is not always 0 for (a+b) * (a-b). I find it hard to grasp that the magnitudes are what made them orthogonal.
  8. Sep 4, 2012 #7


    Staff: Mentor

    True. If you form a parallelogram with a and b as two adjacent sides, then a + b will be a diagonal, and a - b will be the other diagonal. Most of the time these diagonals won't be perpendicular, so the dot product (a + b)##\cdot##(a - b) won't be zero.
    That's because you're essentially working with unit vectors, and the parallelogram is actually a square. In that case, the diagonals are perpendicular.

    |b|a + |a|b = |a| * |b| * (a/|a| + b/|b|)

    |b|a - |a|b = |a| * |b| * (a/|a| - b/|b|)

    The vectors in the parentheses on the right sides are unit vectors, with a/|a| + b/|b| being one diagonal of a unit square, and a/|a| - b/|b| being the other diagonal.

    (Here a/|a| means 1/|a| * a, of course. I'm also using just a single pair of | vertical bars to denote vector magnitude.)
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