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sin^4α - cos^4α=(sin²α - cos²α)(sin²α + cos²α)=(sin²α - cos²α)·1=
1-2cos²α=1-17/2=-15/2
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sinα·cosα = 1/4 ⇒ sin²α·cos²α = 1/16
sin⁴α+ cos⁴α=(sin²α + cos²α)²-2sin²α·cos²α=1-1/8=7/8
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