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sin²α + cos²α = 1
cos²α = 1 - sin²α = 1 - (-1/3)² = 1 - 1/9 = 8/9 ⇒ cosα = √8/3 = 2√2/3 ∉ zał. lub cosα = -√8/3 = -2√2/3 ∈ zał.
Odp.: cosα = -2√2/3