If α∈ (0, π) and cos α + sin α = - 13, then Cos2 α = () A. 179B. ±179C. −179D. 173

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• 1. If α ∈ (0, π) and cos α + sin α = - 1 / 3, then Cos2 α=
• 2. Sin α + sin β = 1. Cos α + cos β = 0 find Cos2 α + Cos2 β
• 3. If α ∈ (0, π), sin α + cos α = 1 / 3, find Tan α, Cos2 α, etc!
• 4. If α belongs to (0, π) and cos α + sin α = - 1 / 3, then Cos2 α =?
• 5. If θ is the second quadrant angle and cos θ / 2-sin θ / 2 = √ (1-2 sin θ cos θ), then what quadrant angle is θ / 2
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• 8. Cos2 β = - (1 / 2), then sin β=
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• 10. Simplification: 1, sin (x - π / 3) - cos (x + π / 6) + √ 3cosx =? 2, known, sin α + sin β = √ 2 / 2, find the value range of cos α + cos β
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• 14. Cos 2 α = - 3 / 5, α ∈ (π / 2, π), sin β = - 12 / 13, the third quadrant of β, find cos (β - α) Delete Cos2 α = - 3 / 5 and change to cos α = - 3 / 5
• 15. If sin β = - 5 / 13, α∈ (π / 2, π), then Cos2 α=
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• 19. It is known that a quarter of the faction is less than a half of the faction, and sin AFA + cos AFA = seven fifths, so cos AFA sin AFA can be obtained
• 20. Sin (a + b) = 4 / 5, sin (a-b) = - 3 / 5, then Tana / tanb=