Posted by **Anonymous** on Thursday, January 20, 2011 at 2:55am.

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

- trig -
**Henry**, Friday, January 21, 2011 at 7:13pm
cosA = -4/5 = -0.80,

A = 143.1deg.

sin(2A) = 2*sinA*cosA,

sin2A = 2*sin(143.1)*cos(143.1 =

-0.9603.

- trig -
**Henry**, Friday, January 21, 2011 at 7:38pm
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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