已知sin(x-π/4)=-1/3,求sin2x的值

问题描述:

已知sin(x-π/4)=-1/3,求sin2x的值
为什么sin2x=cos(2-π/4)?

sin2x
=cos(π/2-2x)
=cos(2x-π/2)
=cos[2(x-π/4)]