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Homework Help: Dot Product

  1. Jan 13, 2016 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
    I've expanded p.(q+r) to give p.q+p.r. The magnitude of p is 3, and since ABE is an equilateral triangle, the magnitude of q is also 3, right? So then p.q=9, but the answer scheme states that p.q=4.5.
    I'm still pretty new to mathematics, so I'm probably missing something really obvious, but it's better to make the mistakes now than in my exam!
  2. jcsd
  3. Jan 13, 2016 #2


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    Science Advisor
    Homework Helper

    p.q is certainly not equal to 9.

    Maybe you could start by stating the geometric definition of the dot product in Euclidean space, or ##\mathbb R^3## in this case.
    Then you will be able to deduce from that the values of p.q and p.r
  4. Jan 13, 2016 #3
    Ah, I've got it now. Thanks for your help!
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