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If sinA=1/2 then find cos^3A-3cosA.

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  1. if sin A = 1/2 then cos A = (sqrt 3)/2
    because that is a 30, 60, 90 triangle
    (1/8) 3 * 3^1/2- (3/2) * 3^1/2

    = -(9/8) * 3^1/2

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