sin（π/4+α)=1/4,则sin2α=?
问题描述：
sin（π/4+α)=1/4,则sin2α=?
答
a=arcsin(1/4)-π/4
sin2a=sin[2arcsin(1/4)-π/2]
arcsin(1/4)=22.5
所以sin(45-45)=sin0=0
答
sin2α
= -cos(2α+π/2)
= -cos2(π/4+α)
= -[ 1-2sin²（π/4+α)]
= -( 1-(1/4)² )
= -15/16