cos2x/sin(x-π/4）=sin(π/2-2x)/sin(x-π/4）=-sin(2x-π/2)/sin(x-π/4）=-2sin(x-π/4）cos(x-π/4）/sin(x-π/4）=-2cos(x-π/4）=-根号2/2则cos(x-π/4）=√2/4sin(2x)=cos(π/2-2x)=cos(2x-π/2)=2cos²(...