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Homework Help: Vectors Using Cosine or Sin Law

  1. May 21, 2012 #1
    1. The problem statement, all variables and given/known data
    Need help with a question ... .__."

    q: if the magnitude of a=2 and the magnitude of 5a-2b= 7.7, and the angles between the vectors a and b is 50 degrees determine the magnitude of b.

    2. Relevant equations

    3. The attempt at a solution

    first I drew a diagram... then i used cosine law to solve for b

    l5a-2bl = (5x2) + (2b)^2 -2(5x2)(2b)cos50

    7.7= 10 +4b^2 -40b cos 50

    this worked out to be a quadratic, the zeros i found were 6.3 & 0.09, but then i subbed those numbers back into the cosine law i don't get back 7.7...

    I tried this question like this, l5a-2bl = (5x2)^2 + (2b)^2 -2(5x2)(2b)cos50, but it didn't work out either.

  2. jcsd
  3. May 21, 2012 #2
    Why not just use algebra to solve for b?
  4. May 21, 2012 #3
    Magnitude of vectors are scalar quantity. Operations just like ordinary real numbers.
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