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Korzystamy z jedynki trygonometrycznej:
sin²α + cos²α = 1
sin²α = 1 - cos²α = 1 - (2/3)² = 9/9 - 4/9 = 5/9
2sin²α - 1 = 2 * 5/9 - 1 = 10/9 - 9/9 = 1/9
Odp. D.