rt 求助

顶

2sin(π/4+α)sin(π/4-α)
=cos(π/4+α-π/4+α)-cos(π/4+α+π/4-α)
=cos2α=1/3;2α是第四象限角;
sin2α=-2√2/3;
sin4α=2sin2αcos2α=-4√2/9;