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Math Help - trigonometry

  1. #1
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    trigonometry

    Given that sin A= 3/5 and cosB= 12/13 where A is obtuse and B is acute, find the exact values of cos(A+B) and cot(A-B)

    I am really having trouble understanding this question, i dont even know how to start..?

    Id appreciate the help.
    Thanks
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  2. #2
    MHF Contributor red_dog's Avatar
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    If A is obtuse then \cos A<0\Rightarrow\cos A=-\sqrt{1-\sin^2A}.

    If B is acute then \sin B>0\Rightarrow\sin B=\sqrt{1-\cos^2B}.

    Then use the identities \cos(A+B)=\cos A\cos B-\sin A\sin B

    and \cot(A-B)=\frac{\cot A\cot B+1}{\cot B-\cot A}
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  3. #3
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    How do we find the values of Sin B and Cos A?
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  4. #4
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    Hello Oasis1993
    Quote Originally Posted by Oasis1993 View Post
    How do we find the values of Sin B and Cos A?
    Have you really looked at red_dog's reply?

    Grandad
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