用诱导公式：cos(2x + π/2 - π/4)=cos[π/2 + (2x - π/4)]=-sin(2x - π/4)
cos(2x+π/4)=cos2x·√2/2-sin2x·√2/2=-sin（2x-π/4）-cos(2x+π/4)=-(cos2x·√2/2-sin2x·√2/2)=sin2x·√2/2-cos2x·√2/2=sin(2x-π/4)
