lim【x→π/4】(sin2x-cos2x-1)/(cosx-sinx)=lim【x→π/4】(2sinxcosx-2cos²x)/(cosx-sinx)=lim【x→π/4】2cosx(sinx-cosx)/(cosx-sinx)=lim【x→π/4】-2cosx=-2cosπ/4=-√2