sin(x-3π/4)cos(x-π/4)=-1/4
-1/4=[sin(x-3π/4+x-π/4)+sin(x-3π/4-x+π/4)]/2
sin(2x-π)+sin(-π/2)=-1/2
-sin2x-1=-1/2
sin2x=-1/2
cos4x=1-2(sin2x)²
=1-2(-1/2)²
=1/2
应用积化和差公式与倍角公式.