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Finding an angle between two vectors

  1. Nov 12, 2009 #1
    1. The problem statement, all variables and given/known data

    Find the angle between the vectors u=[4 -4 -3] and v=[2 2 -3].

    2. Relevant equations

    theta = arccos(u . v)/(|u||v|)

    3. The attempt at a solution

    arccos[(8+(-8)+9)/(sqrt(41)*sqrt(17))]= 70.068 degrees. According to the online homework, it is wrong...and I and stumped.
    Any help would be appreciated!

  2. jcsd
  3. Nov 13, 2009 #2
    Check if their answer isn't perhaps in radians.
  4. Nov 13, 2009 #3
    Well the thing is, it is a webwork assignment, so you can't really check the answer. And the question was not specfic for degrees or radians.:frown: Well, what counts is that I know I did it right. Thanks phyzmatix
  5. Nov 13, 2009 #4
    As a rule of thumb (well, this definitely holds for where I'm studying anyway) angles are ALWAYS in radians, unless specifically stated otherwise. It took me a while to get used to this also as we never did radians in school.
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