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It is known that cos (α + π / 4) = 3 / 5, π / 2 ≤ α
∵ sin α = 3 / 5, α ∈ (π / 2, π) ∵ cos α = - √ (1-sin & # 178; α) = - 4 / 5, so sin (2 α) = 2Sin α cos α = 2 (3 / 5) (- 4 / 5) = - 24 / 25 cos (2 α) = cos & # 178; α = (- 4 / 5) & # 178; - (3 / 5) & # 178; = 7 / 25 Tan (2 α) = sin (2 α) / cos (2 α) = - 24 / 25) / (7 / 25) = - 24 / 7
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