# trig

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

• trig -

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

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

• trig -

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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