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Find values so that the vector has the specified angle

  1. Feb 1, 2015 #1
    1. The problem statement, all variables and given/known data
    Find the values of x such that the angle between the vectors <2, 1, -1>, and <1, x, 0> is 45 degrees.

    2. Relevant equations
    <2, 1, -1> dot <1, x, 0> = 2 + x
    |first vector| = (6)0.5
    |2nd vector| = (1 + x2)1/2
    Angle = cos-1(a dot b)/(|a||b|)
    Cosine of 45 is the root of two divided by two.

    3. The attempt at a solution
    I set (2 + x) divided by 6^0.5 and (1 + x2)^0.5 equal to 1 divided by the root of 2.
    I eventually get 2√2 + x√2 = √6√1 + x2
    Am I on the right track?
  2. jcsd
  3. Feb 1, 2015 #2


    Staff: Mentor

    Or equivalently, ##\frac{2 + x}{\sqrt{6}\sqrt{1 + x^2}} = \frac{\sqrt{2}}{2}##
    I'm not sure about the above, although it might be right. I get a quadratic with all integer coefficients.
    Can you show what you did going from your equation above to this one?
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