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Trig question confusion.

  1. Dec 4, 2013 #1
    Not homework just my own revision.

    If sin a = –cos b = 3/5 and a and b are both in the second quadrant, what is cos (a – b)?

    Now keep getting the answer 0, but the answer is apparently 24/25, now they use the trig subtraction formula, I just did cos ((arcsin(3/5) - arccos(-3/5)) I got 0 as the answer.

    Was my method wrong, where did I go wrong with it?
  2. jcsd
  3. Dec 4, 2013 #2


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    Science Advisor

    Both arcsin and arccos are multivalued. You need to get the possible solutions and see which are in the second quadrant.
  4. Dec 4, 2013 #3
    Ah that's what the explanation was talking about.
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