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Thread: Trig Difference Identity: when I do sin(-15) = sin(30-45), I get a positive result

  1. #1
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    Trig Difference Identity: when I do sin(-15) = sin(30-45), I get a positive result

    Hello,

    I am trying to find sin(-15). Do the difference identities apply to negative angles? I am struggling to come up with the correct answer because when I do sin(-15) = sin(30-45), I get a positive result. Can someone please explain the flaw in my logic?

    Thanks!

  2. #2
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    sin(-15) does not equal sin(15), just look at the graph of sin(x).
    Edit: please disregard, misunderstood the question. Yes, your approach should work. How did you evaluate sin(30-45)?
    Last edited by lev888; 01-30-2018 at 11:19 AM.

  3. #3
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    Quote Originally Posted by BigNate View Post
    Hello,

    I am trying to find sin(-15). Do the difference identities apply to negative angles? I am struggling to come up with the correct answer because when I do sin(-15) = sin(30-45), I get a positive result. Can someone please explain the flaw in my logic?

    Thanks!
    It should work for any angle; can you show the details of your work, so we can find the flaw?

    My guess is that you either used the wrong formula, or made a little sign error.
    Last edited by Dr.Peterson; 01-30-2018 at 01:20 PM.

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