Q.
If cosβ is the geometric mean between sinα and cosα where 0<α,β<π/2 then cos2β is equal to
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a
−2sin2π4−α
b
−2cos2π4+α
c
2sin2π4+α
d
2cos2π4−α
answer is A.
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Detailed Solution
2sinαcosα=2cos2βsin2α=1+cos2β∴ cos2β=−(1−sin2α) =−1−cosπ2−2α =−2sin2π4−α =−2cos2π4+α
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