sin（π/4+α)=1/4,则sin2α=?
问题描述：
sin（π/4+α)=1/4,则sin2α=?
答
sin2α
= -cos(2α+π/2)
= -cos2(π/4+α)
= -[ 1-2sin²（π/4+α)]
= -( 1-(1/4)² )
= -15/16