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Trigonometry, find cos(a-b)

  1. Jun 18, 2011 #1
    1. The problem statement, all variables and given/known data

    Given that
    sin(a) + cos(b) = 31/2/2 and
    sin(a) + sin(b) = 3/2

    Find cos(a-b)

    2. Relevant equations



    3. The attempt at a solution

    Can anyone give me some hints?
     
  2. jcsd
  3. Jun 18, 2011 #2

    ehild

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    See: http://file.glpacademy.co.kr/eTAP/mathfiles/english/trigo/lesson2/instructiontutor_last.html [Broken]

    ehild
     
    Last edited by a moderator: May 5, 2017
  4. Jun 18, 2011 #3

    eumyang

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    Try taking the Pythagorean identity
    cos2 b + sin2 b = 1

    and rewriting it in terms of sin a. I found an answer by starting this way, but it is a decimal approximation.
     
  5. Jun 18, 2011 #4
    The answer i get is 0.7858... but the answer given is 0.5. What answer do you guys get?
     
    Last edited: Jun 18, 2011
  6. Jun 19, 2011 #5

    eumyang

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    0.7858... is what I got, too. Maybe there is a typo in the original problem or in the answer?
     
  7. Jun 19, 2011 #6

    I like Serena

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    I get a total of 2 solutions: 0.2599 and 0.7858
     
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