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Math Help - sums and differnces of different angles

  1. #1
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    sums and differnces of different angles

    the question is:
    find the exact value of
    sin(x-y) when sinx=8/17 and cos=5/13

    need help
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  2. #2
    MHF Contributor red_dog's Avatar
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    Use the formula \sin(x-y)=\sin x\cos y-\sin y\cos x.
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  3. #3
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    Quote Originally Posted by red_dog View Post
    Use the formula \sin(x-y)=\sin x\cos y-\sin y\cos x.
    but you only have sinx and cosy you dont have siny and cosx
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  4. #4
    MHF Contributor red_dog's Avatar
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    \cos x=\pm\sqrt{1-\sin^2x}, \ \sin y=\pm\sqrt{1-\cos^2y}

    To choose the sign before the radical, you have to know in which quadrant are x and y.
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  5. #5
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    i wasnt taught the radical yet, is there another way like guess and check?
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  6. #6
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    Quote Originally Posted by pogiphilip View Post
    i wasnt taught the radical yet, is there another way like guess and check?
    Hi

    Do you know the domain for x and y ? ie 0<x<90 ??
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