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Homework Help: John Taylor Classical Mechanics Chapter 1 Problem 22

  1. Aug 23, 2013 #1
    1. The problem statement, all variables and given/known data
    The two vectors a and b lie in the xy plane and make angles (alpha and beta) with the x axis. a. by evaluating a dot b in two ways prove the well known trig identity cos(alpha-beta)=cosalphacosbeta +sinalphasinbeta

    2. Relevant equations

    3. The attempt at a solution
    Could not really come to the proper set-up.
  2. jcsd
  3. Aug 23, 2013 #2


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    How are the x,y components of a vector related to the angle they make with the x axis?

  4. Aug 23, 2013 #3
    the angle they make is in the problem; the cos of the difference of the angles equals that result. i just cud not go from the dot product to that.
  5. Sep 6, 2013 #4


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    hi karmonkey98k! :smile:

    start with a = |a|cosαi + |a|sinαj :wink:
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