Q.
If cos β is the geometric mean between sin α and cos α, where 0<α, β<π/2, then cos 2β 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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