y=4sin(x-π/4)cos(x+π/4)
=4sin(x-π/4)sin[π/2-(x+π/4)] /后面一项运用余角公式
=4sin(x-π/4)sin(π/4 -x)
=-4sin²(x-π/4) /sin(-a)=-sina
=-2[1-cos(2x-π/2)] /运用二倍角公式：2sin²a=1-cos(2a)
=-2[1-cos(π/2 -2x)] /cos(-a)=cosa
=-2[1-sin(2x)]
=2sin(2x) -2