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trig

cos A = -4/5 for angle A in Quadrant II, find sin 2A

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  1. cosA = -4/5 = -0.80,
    A = 143.1deg.

    sin(2A) = 2*sinA*cosA,
    sin2A = 2*sin(143.1)*cos(143.1 =
    -0.9603.

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  2. Solution using Exact values.

    cosA = -4/5 = x/r.

    (-4)^2 + Y^2 = 5^2,
    16 + Y^2 = 25,
    Y^2 = 25 - 16 = 9,
    Y = 3.

    SinA = Y/r = 3/5.

    sin2A = 2 * 3/5 * -4/5 = -24/25.

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