sinAsin(120°-A)=sinA（sin120°cosA-cos120°sinA）=√3/2sinAcosA+1/2sin²A=√3/4sin2A+1/4-1/4cos2A=1/4+1/2(√3/2sin2A-1/2cos2A)=1/4+1/2(cos30°sin2A-sin30°cos2A)=1/4+1/2sin(2A-30°)