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Posted by Moe on Saturday, April 27, 2013 at 9:51pm.

If cos y = -4/5 when 90°≤ y ≤ 180° what is the value of cos 2y?

quadrant 2 cos 2y = 1 - 2 sin^2 y but sin y = 3/5 (3 4 5 triangle) cos 2 y = 1 - 2 (3/5) = 1 - 6/5 = -1/5

last 3 lines should be cos (2y) = 1 - 2(3/5)^2 = 1 - 18/25 = 7/25 or .28

Whoops, thank you

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