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Homework Help: Trig' Identities - Addition Formulae

  1. Sep 19, 2010 #1
    [STRIKE]1. The problem statement, all variables and given/known data

    Given that cos C = 12/13 where C is a reflex and sin D = 3/5 where D is acute, find the exact value of cos ( C + D ).

    2. Relevant equations

    3. The attempt at a solution

    I used the Addition Formulae: cos(A+B) = cosAcosB-sinAsinB

    cos(12/13)cos(2/5) - sin(1/13)sin(3/5) = 0.999831....

    As it says "exact value" I assume my long decimal answer is incorrect. I have inputed it all straight into my calculator. Help me please :smile:[/STRIKE]

    Sorry - I've worked out my silly mistake. Thank you for looking :smile:
    Last edited: Sep 19, 2010
  2. jcsd
  3. Sep 19, 2010 #2
    First off, you're not looking to find

    cos(12/13)cos(2/5) - sin(1/13)sin(3/5)

    you want to find

    cos(C)cos(D) - sin(C)sin(D).

    You are given the values of cos(C) and cos(D) in exact for. So your next step is to find the exact values of sin(C) and sin(D). There is one identity that is helpful here, can you think of which one?
  4. Sep 19, 2010 #3
    Thank you for the help JeSuisConf but I've spotted the error.. Silly me, was making it far complicated than it actually is..

    Thank you for looking though :smile:
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