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Math Help - Find sin (x + y)

  1. #1
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    Find sin (x + y)

    Angle x is acute, angle y is obtuse, cos x = 15/17
    and sin y = 3/5. Find sin (x + y).

    After doing the math, I found sin (x + y) to be 77/85.

    The textbook tells me that the answer is 13/85.

    I know 13/85 would be the answer if we were looking for
    sin (x MINUS y), right?
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  2. #2
    MHF Contributor Calculus26's Avatar
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    sin(x+y) = sin(x)cos(y) +sin(y)cos(x)

    Since angle y is a second quadrant angle cos(y)< 0 so you use cos(y) = -4/5 instead of 4/5

    sin(x+y) = (-32+45)/85
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  3. #3
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    ok...

    Quote Originally Posted by Calculus26 View Post
    sin(x+y) = sin(x)cos(y) +sin(y)cos(x)

    Since angle y is a second quadrant angle cos(y)< 0 so you use cos(y) = -4/5 instead of 4/5

    sin(x+y) = (-32+45)/85
    I now see what I did wrong. Thanks.
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