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cosα / sinα = 4
cosα = 4sinα
cos²α = 16sin²α
sin²α + cos²α = 1
sin²α + 16sin²α = 1
17sin²α = 1
sin²α = 1/17
cos²α = 16sin²α = 16 × 1/17 = 16/17
sin²α - cos²α =
= 1/17 - 16/17 =
= -15/17