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Angle x is in the first quadrant and angle y is in the second quadrant, such that Cosx = 12/13 and Siny = 7/25. Determine an exact value for Cos(x+y).

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  1. If you draw the right triangles for x and y in standard position, you can see that
    sinx = 5/13
    cosy = -24/25
    Now just use your sum formula
    cos(x+y) = cosx cosy - sinx siny
    = (12/13)(-24/25) - (5/13)(7/25) = -323/325

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    oobleck

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