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Homework Help: Finding the length of a vector given the magnitudes

  1. Feb 6, 2012 #1
    1. The problem statement, all variables and given/known data
    The question is as follows: if |m| = 4, |n| = 3, and the angle θ between m and n is 5pi/6, find the norm of the vector m + 2n.

    2. Relevant equations

    Im attempting to use the equation :
    cosθ = (m * n)/(|m||n|)

    3. The attempt at a solution

    I determined using the above formula that the value of m * n is 6√(3), and is either negative using the given 5pi/6 or positive if you use the reference angle of pi/6. I am not entirely sure how to relate that value to the length of m +2n. Would you just multiple |n| by 2 in that formula? which would yield 12√3. Again not sure if I can relate these 2 things, thanks
  2. jcsd
  3. Feb 6, 2012 #2


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    The norm of vector, v is is given by: |v| = √(vv).

    So, look at (m + 2n)∙(m + 2n)
  4. Feb 6, 2012 #3
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