If 0 < α < π, 2sin2 α = sin α, then cos (2 α - π / 2)=
2sin2a = Sina, 4sinacosa = Sina, so cosa = 1 / 4, Sina = √ 15 / 4
cos﹙2α﹣π/2﹚=sin2a=2sinacosa=2*1/4*√15/4=√15/8
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