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Math Help - Vector dot products problem

  1. #1
    Junior Member
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    Oct 2008
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    Vector dot products problem

    If 'a' and 'b' are unit vectors and |a+b| = sqrt(3)
    what is (2a-5b) (b + 3a)?
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  2. #2
    Member
    Joined
    Aug 2008
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    Hello,

    aa=bb=1.
    (a+b)(a+b)=3 implies aa+2ab+bb=3.
    Once you know ab, substitute in (2a-5b)(b + 3a)=2ab+6aa-5bb-15ab.

    Bye.
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