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sinα=⁵/₇
(⁵/₇)²+cos²α=1
cos²α=1-²⁵/₄₉
cos²α=²⁴/₄₉
cosα=√(²⁴/₄₉) ∨ cosα =-√(²⁴/₄₉)
dla α>90°
cosα jest ujemny
odp B.