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sin²α+cos²α=1
(1/9)²+cos²α=1
cos²α=1-1/81
cos²α=80/81
cosα= -√80/9= -4√5/9
sin²α + cos²α = 1
cos²α = 1 - sin²α = 1 - (1/9)² = 1 - 1/81 = 80/81
cos²α = 80/81 ---> cosα = √(80/81) ∨ cosα = -√(80/81) wybieramy ten z minusem, bo II ćwiartka.
cosα = √(80/81) = -4√5 / 9
GOTOWE!!!