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Homework Help: Vector Proof

  1. Jun 21, 2012 #1
    1. The problem statement, all variables and given/known data

    Given vector a and b, let vector v = (|a|b + |b|a) / (|a| + |b|). Show that vector v bisects the angle created by vector b and a.

    2. Relevant equations

    cos(θ) = a dot b / (|a||b|)

    3. The attempt at a solution

    I used the dot product equation to find angle between a and v.

    cosθ = (a dot v) / |a||v|

    i substituted ((|a|b + |b|a) / (|a| + |b|)) for v.

    cosθ = (a dot ((|a|b + |b|a) / (|a| + |b|)) / |a||v|

    I distributed the (a dot ((|a|b + |b|a)

    cosθ ( ((|a|b dot a) + (|b| |a|^2) ) / (|a| + |b|)) / (|a||b|)

    I then tried to get the cosθ of b dot w to the same thing. I tried to get the two angles equal to each other. My idea is that if the angles formed by a and v and b and v are equal then v bisects the two angles.
  2. jcsd
  3. Jun 21, 2012 #2


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    So you have
    $$\cos\theta_{av} = \frac{a(\vec{a}\cdot\vec{b})+a^2b}{(a+b)av}$$ where ##a=|\vec{a}|##, ##b=|\vec{b}|##, and ##v=|\vec{v}|##, and the corresponding expression for ##\cos\theta_{bv}##. So far so good. Try calculating
    $$\frac{\cos\theta_{av}}{\cos\theta_{bv}}$$ and show it's equal to 1.
  4. Jun 21, 2012 #3

    The problem becomes easy when you realize that if you add two different vectors of equal lengths, then you get a third vector whose direction bisects the angle of the first two vectors. Try it and see. That takes care of the geometry. Now see if you can figure out the algebra.

    Last edited: Jun 22, 2012
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