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Homework Help: Find cos 2A

  1. Aug 15, 2009 #1
    1. The problem statement, all variables and given/known data
    Find Cos2A if sinA= -1/3 and pi< (or equal to) A < (or equal to) 3pi/2

    2. Relevant equations

    3. The attempt at a solution
    Do i use sin-1 to find when sin equals -1/3
  2. jcsd
  3. Aug 15, 2009 #2


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    Gold Member

    Yes, you can do that, that's the simple way. Take note of where your answer occurs as there are 2 points where sin(x) = -1/3. The question tells you the domain of A.
  4. Aug 15, 2009 #3

    D H

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    Staff Emeritus
    Science Advisor

    That's the hard way. The easy way is to use the a identity that expresses cos(2A) in terms of sin(A).
  5. Aug 16, 2009 #4
    You should use the identity cos2A = cos2A - sin2A and

    cos2A = 1 - sin2A

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