Trig question confusion.

  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. mathman

    mathman 6,471
    Science Advisor
    Gold Member

    Both arcsin and arccos are multivalued. You need to get the possible solutions and see which are in the second quadrant.
     
    1 person likes this.
  4. Ah that's what the explanation was talking about.
     
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