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Book demonstration about trigonometric relations

  1. May 9, 2017 #1
    1. The problem statement, all variables and given/known data
    a7d2c0139ac54fd0a5181cade3546078.png


    In the equation between [itex](3)[/itex] and [itex](2)[/itex], why does the author says that 4853327b00864221b28222f8ec109261.png ? Isn't the trigonometric identity actually b1d600bf2773406fb9443fa8e18b6fac.png ?

    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. May 9, 2017 #2

    FactChecker

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    You are right, but that doesn't mean that what they say is wrong. In fact it proves that their statements are correct. They want to prove something about the absolute value, so they took the absolute value of both sides.
     
  4. May 10, 2017 #3
    Sorry, I don't understand what you mean. How does it prove his statement is correct?

    I wasn't wondering about the absolute values, I was confused about the fact that it should be two times the cosine, not the sine.
     
  5. May 10, 2017 #4

    FactChecker

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    It's the same thing. Multiplication is commutative. They just swapped the order of the multiplication.
     
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